By the end of this program, it is my hope that you will not only understand solar design, but also be much more comfortable talking about it with friends, family and clients when incorporating solar into your projects. My name is John Cromer, and I am an Ivy League mechanical engineer. I have been working in all aspects of solar, from teaching continuing education, to designing residential and commercial systems, to now installing offgrid smart homes, with twelves years of industry experience.
It might surprise you to learn that I am a Texan who left the oil industry where I was doing control system engineering and contract management in order to pursue a career path in renewable energy in general, and solar in particular. We are in an expanding industry, one which will substantially change our power grid, and I hope that you too will decide to get involved!
Let’s dive right in! As we talk about solar, I think the most important thing to keep in the back of your mind is that the value of solar power has less to do with how much sunlight is available where you plan to locate an array and more to do with how much that electricity is worth. For example, the desert in the southwest United States gets great sunlight while the Northeast gets comparatively little.
People think that more solar arrays would exist in very sunny places. Not so. Places like Germany, and the Northeast of the United States have far more solar arrays than what you will find in a desert, because solar is more valuable where the price of electricity is very expensive. I happen to be working a lot in Mississippi and other southern states. Mississippi has plenty of sunlight year round. There is very little, if any, snow in the deep South. The price of electricity is average.
However, this brings us to our second point, which is about the how much solar electricity is worth, or more specifically, the grid buyback rate. Sometimes referred to as net-metering, this is a very important variable for how valuable solar is to an array owner. Net-metering is basically about how much you get from your power company when your solar array pushes its excess solar energy onto the grid.
If you are trying to offset 100% of your energy use, perhaps 2/3rds of that production will be pushed onto the grid. After all, solar only produces during the day, but we use electricity in the morning, evening, and at night as well. So offsetting 100% of your energy use is not the same as eliminating 100% of your electric bill, and it has to do with your buyback rate.
When a Mississippi solar array owner backfeeds her excess electricity, she only gets back about 20% of retail value of electricity. Other states have much higher buyback rates, sometimes near full retail value. Since net-metering policy is determined by each individual state, what drives the solar industry is not so much the amount of sunlight available, but rather the raw cost of electricity in a region as well as how the utility compensates the user for back-feeding onto the grid.
There is a third, more nuanced issue regarding how users are billed for the electricity that they use from the grid, which requires that understand our electric bill. We’ll get to that later on in the program.
It all comes down to money, so first things first: let’s do a budget review.
This is a real project that I did at the end of 2018, using the components which fit into a design aesthetic focused on both cost-effectiveness and future-proofing, which is ultimately completed in the residential off-grid section of the program. For example, I upgraded the inverter to a lithium ion battery inverter, even though I did not include a lithium ion battery which could be added later. Perhaps my favorite standard upgrade from traditional solar is to use “all black” modules, even though these aren’t necessarily the most expensive or top shelf solar panels.
I am still seeking a cost-effective solar panel, but paying a bit more for all black panels results in a more aesthetically-pleasing rooftop, which is important for resale value. I use internal cable runs through the attic, as well as fire-code friendly and shade tolerant “module-level panel electronics”, with a small amount of additional infrastructure for system expansion.
In other words, this is not the cheapest system you could possibly do, nor is it the most expensive. In all projects, we must be razor-focused on budget, especially because they do not have good solar policy. Even so, I think it is worth spending a little more on small upgrades compared to limiting the system to the lowest possible budget. Hopefully you’re interested in learning what those things are throughout the class.
So, these are real, hard installed costs which you can achieve on a quality, battery-less solar array while even allowing for some future expansion.
This was a 14 kilowatt array, comprising of two “pallets” of solar panels – another cost-cutting measure is to design in pallet quantities, as you get price breaks when ordering and shipping panels which have not been broken out from their manufacturer’s shipping containers. Maybe I could have fit a few more solar panels onto the roof, but instead I focused on an aesthetically pleasing design with two pallets worth of solar panels.
In the solar industry, we price by the watt, in a similar fashion to how a home builder prices by the square-foot. For this project, I achieved a total budget of $2.50 per watt. But if we look at what the national average pricing is for residential installed solar in 2018, there’s a very different picture. Some states do not have much competition, unlike crowded solar markets like New York or California, but rather the solar policy is so bad, the project budget is forced low to achieve minimally acceptable project economics for the end user.
We can see here that most residential solar installed in the United States hovers at a little over $3 per watt installed pricing. However, depending on your state, solar could be cheaper. In other states, it doesn’t have to be cheap to still be cost-effective and installers aim for higher margins and more expensive system components. Finding the right balance for your client is why there is opportunity in solar design and project development.
Let’s take a look at what goes into solar pricing. This particular chart says 2015 and 2016 but it’s still pretty good data. While solar has gotten cheaper, there have also been import tariffs which have kept the price of solar panels and inverters high. In dark blue we see the solar panel itself hovering around $0.45 to $0.50 per watt including the import tariffs, at the residential level, whereas the solar inverter is a little bit less than that, maybe around $0.30 cents per watt. These prices can be cheaper for low-end systems or more expensive when you get into battery components.
Here is your electrical Balance-of-System material that you should not get too cheap on, as there’s some nice things you can do with balance-of-system material selection. We’ll be talking about in class that really preserves system value, such as internal cable runs to keep the array looking nice. Most clients want their systems looking clean and polished. Minor upgrades will not increase your cost exorbitantly so it doesn’t cost that much more to achieve a high end look, regardless of what solar panels or inverter system you use.
Next, let’s talk about direct labor cost. Direct labor cost is the take-home pay of the installer, excluding things like profit, overhead, and supply chain markup. So direct cost is what the guys out in the field are taking home in their wallets at the end of the project, not necessarily the total pay of the development company, sales rep commission, or project manager salary. Racking is around $0.20 a watt. Design, engineering and permitting is a modest cost.
The reset of the budget, which is pretty much half of your residential project cost, is called, “soft cost” which includes profit, overhead, and supply chain markup. That’s also your sales commission, plus the 30% margin that the developer is charging to manage the project, do the construction, and other tasks to put it all together. So if you can get rid of the soft cost.
If you are an ambitious do-it-yourselfer with some electrical competency, wanting to add solar to your own roof as a hobby, you might only pay a little bit in supply chain markup and see your cost drop below $2 a watt, even with enough budget to hire some qualified labor. You could also go through a competitive bid process and see some of these margins reduce. And by the end of the program, you will be able to do many project scope items such as array layouts, material lists, and performance estimation yourself.
Obviously, getting multiple quotes is a good idea. Last year, an online sales company targeting southern states, was selling above the $4 per watt for very basic systems, while grossly misrepresenting the economics and functions of batteryless solar to the customer. These customers would have saved both money and heartache if they had simply gotten local pricing. It’s always good to have on your bid list a local installer who is more likely to be more knowledgeable of local buyback rates and utility solar policies.
So far so good? One of the greatest challenges of doing a solar project is nailing down all the odds and ends. But by the end of this solar class, you should know enough to assemble a material list necessary to streamline the ordering process from online solar distributors without much design work needed on their end. This ultimately is what results in the cheapest solar project, regardless if it is top shelf, bargain bin, installed, or a DIY project.
Let’s move onto performance estimation. How much power a solar array will produce in your area? First let’s define some terminology. Here is a solar panel rated for 250 watts, which is a little bit small compared to what is being installed today. The greatest efficiency panels, which would be the same size but be more energy dense, can go all the way up to 350 watts, but they’re also much more expensive.
I typically recommend high efficiency panels in markets with limited, expensive real estate, such as New York City, or in other areas where there is very little room to put panels. The panels I commonly install are around 300 watts. If you are in a less densely populated area with easy rooftops and plenty of “real estate” for the roof or ground mounting, you can lower your pricing by going with a lower efficiency, more cost-effective solar panel. However, in this example, I use 250 watts for mathematical ease.
Remember: if a one kilowatt solar array were exposed to full sunlight for one hour, it would produce one kilowatt hour. That sounds great but in reality a full “sun hour” is only achieved under specific laboratory-controlled conditions, and so that is not what the solar panel will actually do outside. The amount of sunlight in the air varies greatly throughout the day. At high noon there is a lot of sunlight, and in the evening, not so much.
So, how do we figure out how much sunlight is hitting the panel? What about temperature impact or humidity? These too can impact your solar array performance. So how do we actually get a good idea of what our solar array will produce? Thankfully, there is a free software put out by the Department of Energy called PVWatts and it’s a very good starting point. In fact, I even use PVWatts to go all the way into off-grid building design. PVWatts has an almost misleadingly simple interface. I say that because the raw data that PVWatts uses is based on actual weather data in your area and many paid commercial solar design software use the same data sets that feed into PVWatts!
So what I want to do now is a PVWatts example. I want to find out how much energy one kilowatt of solar will produce in a year, measured in kilowatt hours. Most residential solar rates are multiple kilowatts large. I recommend memorizing what one kilowatt of solar will produce in a year, because knowing that ratio creates a scalar you can use to quickly calculate all kinds of off-the-cuff energy estimates with your clients, sounding confident and cool when stating how much energy, say, an eight kilowatt solar array produces a year. In other words, knowing what one kilowatt of solar will produce can be multiplied by eight to get a production estimate for what eight kilowatts of solar will do.
In my region, I know that one watt of a popular utility scale single-axis tracker will produce 1.7 kilowatt hours per year. Multiply by one thousand, a one kilowatt will produce 1.7 megawatt hours per year, or one megawatt will produce 1.7 gigawatt hours per year, and so a 8 megawatt utility scale solar array multiplied by 1.7 will produce 13.6 gigawatt hours of electricity per year.
I also know that for a residential rooftop array, 1 watt will produce 1.4 kilowatt hours per watt per year. So if I determine five kilowatts can fit on a residential project site, I simply multiply 5 kilowatts by 1400 kilowatt hours per year to get 7000 kilowatt hours. Divide that by 12 to get a monthly average, although we will see that summer months produce about twice as much as the winter months. If I’m given an electric bill that says my client’s house uses 14,000 kilowatt hours of electricity a year, and I know 1 kilowatt of solar produces 1.4 kilowatt hours per year, then 14,000 divided by 1.4 results in a 10,000 watt or 10 kilowatt solar array.
What this graphic illustrates is that PVWatts ties in to local weather stations, such as those found on military bases and airports, and it will base its performance estimate off of the most typical month of weather data for each month that has been recorded at that station. The records go back thirty years so it’s a broad data set that results in a solid year-to-year running average of production, although weather anomalies do vary from year to year.
You might be thinking, “If 1 watt will produce 1.4 kilowatt hours, how does the orientation or tilt angle impact system production?” and the answer is “Yes, it does matter, but not as much as you might think”. After all, if you draw a big circle around a rooftop, say, with an imaginary lasso, the same amount of sunlight is going to fall through that lasso, so capturing as much of that sunlight as possible is contained within a maximum limit, at which point it becomes how much surface area can you cover effectively.
So in this example we take Austin, Texas with the theoretical ideal fixed tilt angle for maximum year round production, which is facing due south at the line of latitude. With Austin, Texas being around a 30° line of latitude the ideal tilt angle for Austin Texas is 30° facing due south.
Of course the ideal tilt angle does not factor in how much that electricity worth. If you have a perfect net metering policy and the utility is buying back every single kilowatt hour at retail price, the ideal tilt would produce the most amount of energy and therefore the most amount of money.
But let’s say the utility gives you a higher rate in the evening and a lower rate in the morning, known as time-of-day or time-of-use rate structures. In that case, the most cost-effective tilt angle might not be a line-of-latitude tilt. In reality, it is rare to see a solar array installed at the ideal tilt angle – economic and structural considerations will often result in the optimal tilt being simply to remain parallel to the existing tilt of the roof, known as a flush mounting. An ideal tilt angle might result in more outflow onto the grid, whereas a different angle might result in more of the electricity being consumed on site. So absent a true net-metering policy, the most productive tilt angle is not necessarily the most economic.
If you take a solar panel in Austin, TX at a 30 ° tilt and adjust the tilt up to 50°, you will only lose about 9% of your system production. Likewise if you take that 30° tilt and you tilt it down to a 10° angle you only lose about 9% of your production. And the flatter you go, the less orientation matters because if the module is flat and facing straight up in the air, how it rotates doesn’t matter. In other words, at a shallower tilt angle, the impacts of orientation matter less, although some might advocate for steeper tilt angles to boost winter production. It seems to me that the trend in residential roofing is for steeper roofs, but it’s much easier to install on a shallower roof than a steep roof. Not that you can’t do a steep roof, but if designing a building from scratch, as an installer I’d much prefer a roof with a shallow tilt that you can walk on.
The orientation, whatever direction it faces can impact performance, but much more at steeper angles than shallow angles. If we go from due south to southeast or due south to southwest at a 30 degree tilt, we only lose 3% of the total system production. If I only lose 3% of system production, that will not impact my rough estimate of one watt producing 1.4 kilowatt hours very much. If we stay at a 30 degree tilt and go all the way to due east or all the way to due west, we start to see the performance drop more substantially but even so we’re only losing about 15%. So what we see is that in Austin, Texas an ideal orientation will produce 1370 kilowatt hours a year, whereas a southwest or southeast will produce pretty much the same thing.
There’s an argument to say that a big open west and east roof surface is better than just installing on a small southern roof, due to economies-of-scale. Maybe the individual panels produce 15% less but by doing the larger project we get more than a 15% installation pricing discount. So a large project that is off ideal can be more cost-effective than a small project that only faces ideal because of course only one of the four orientations of your roof surface is going to face south.
Some might even argue that going all the way to a north facing array can be justified depending on your installation price and your cost of electricity. The fact of the matter is, if I’m already up on the rooftop installing a dozen solar panels, moving on to the next roof surface and installing a dozen more solar panels is not going to double my project cost. I just double my solar panel price and I double my direct labor price and then maybe I make a little bit more money from the project but it’s not doubling every single item because it’s not doubling your soft cost. So going back a few slides what we see is that as the projects get larger and we move into commercial scale and utility scale what we see is the soft cost in that light gray margin start to shrink and the project get cheaper, approaching $2 per watt or less.
With an east-facing array or a flat array you’re losing ten to fifteen percent and with a north-facing array you’re losing 30%, but it’s not like there is a hard and fast rule that says the solar panels must face south. Taking what we’ve learned, you could argue covering the whole roof with solar would be best. Covering the whole roof certainly have both economies-of-scale and allow the roof to age in a uniform manner.
At any rate, next we’re going to use PVWatts to do a system performance estimate. When you get to the end screen of PVWatts, you will get a month-by-month printout of how much power the solar array will produce as well as an annual total. The number one mistake you can make is just to end right there, because there’s a whole other aspect of PVWatts – if you download the hourly results you get all the raw data behind the calculations. So, you’d really like to get into the brass tacks of it don’t forget to download that hourly information because that’s where you get the environmental data.
That’s where you get how much solar insolation or array irradiance is in the air and on the module surface. These are synonyms, for a unit of energy measured in watts per meter square. So array irradiance is how much energy is falling across the surface of the solar panel. And, there’s more information than that! There’s direct irradiance and diffuse irradiance which allows you to determine if it’s a cloudy day or a sunny day. We use this in our off-grid designs to tabulate how many days in a row of cloudy weather you get which impacts the size of your battery bank.
Another useful bit of information is how hot the panels get up on the rooftop, something solar designers need to appreciate. Electrical components have temperature ratings as do wire termination lugs. Wire splice points tend to be the hot points of the system. Wire terminations are common when transitioning from the rooftop into the attic. Your boxes and cable ratings inside your house may only have a 75 C or even a 60 C temperature rating, whereas solar cable typically has a 90 C temperature rating. In other words you don’t want to just use any cable that you can buy at Home Depot for wiring up your outdoor roof-mounted solar array. You need to make sure that your boxes, terminals, and cables have a high enough temperature rating. On a 100° F day in Phoenix, Arizona the surface of that solar panel can be a160°F and that impacts your array performance and creates safety hazards for underrated or poorly installed cable terminations. Now, we can identify the maximum temperature of the solar panel on the roof because PVWatts does calculate that. PVWatts does make some assumptions about the data that a commercial software might fine tune, but typically it results in a conservative estimate.
Commercial software will use the actual solar panel that you’re designing the system with in order to perform th energy calculation, whereas PVWatts gives you a couple of values that to switch between, regarding but what is called the temperature coefficients of voltage, current, and power. So what we see is in the this temperature coefficient of voltage is -0.29 percent per degree Celsius. The temperature coefficient of current is a positive 0.06 percentage per degree Celsius that is an order of magnitude less than by how much voltage varies with temperature.
So, as temperature climbs the voltage is going to drop and the current is going to increase. On cold days the opposite will happen.
Data provided on the spec sheet at what is called the standard test condition. The voltage of the module, the amperage of the panel, the combination of the two which is power, all of that data assumes a temperature of 25°C and so especially in your hotter climates, we see from the module temperature climbs substantially above 25°C.
What’s the performance loss between the hot rooftop and the standard test condition? We look at a 25°C rating compared to a 65°C temperature to find a 40° C difference between the two. If our temperature coefficient of power is -0.34 percentage per degree Celsius. Then, we we multiply it by our 40°C temperature difference between the lab condition and what we actually saw up on the rooftop, the result is a 13% energy loss due to heat alone. So, you take the wattage rating of your array and on these hot summer days in Arizona you already know that you’re losing 13% from heat alone, before other factors are considered. One such factor is the friction of the electrons as they travel down the cable, called voltage drop. PVWatts takes all these factors into account to estimate the performance of solar in your area.
I use PVWatts because it’s easy easy to produce a good enough estimate of an unshaded array. In summer you get greater production because of longer days, but you also have the temperature loss due to heat. In the winter it’s colder so you don’t have the temperature loss but the days are shorter. The Sun being further away is not as bright so you don’t have as much energy hitting the surface of the panel. Using pvwatts to understand how the energy is reduced from the solar arrays standard test condition nameplate rating can help you pick the right inverter size for the solar array (We’re going to get into this as the class progresses).
Here’s your standard test condition rating, listed at 1000 watts per meter squared with a module temperature of 25°C. Another factor of standard test condition is called air mass or atmosphere thickness and that’s the hardest one to nail down exactly. It relates to how far through the atmosphere the photon is traveling, and so the atmosphere thickness at noon would be 1. In the morning, the sunlight travels through a greater cross-section of the atmosphere and so you get a higher atmosphere thickness. That value will also be watered down with humidity, clouds, air pollution etc.
So with an air mass rating of 1.5, standard test condition is not modeled at high noon with zero humidity, but assuming mid-morning sunshine with some humidity or high noon with heavy humidity, with a thousand watts per meter square of energy in the air. So, we go back to our PVWatts printout to get more detail of how that energy actually varies with hours of the day, as well as seasonal variations. We can see between 6 – 8 a.m. we we’re only getting up to 500 watts per meter square. Then, around noon we get a little bit over 1000 watts per meter squared. And so, this 1kW solar array is turning on in May at 7 a.m. and staying on past 4 p.m., but it’s not at full power all day. It’s power curve is shaped like a camel’s hump with maximum power occurring midday.
PVWatts redefines the day’s worth of solar insolation into a number based on how many thousand watt per meter square hours the solar array effectively receives, which might be 4 hours a day even though the solar array is energized for 8 hours a day at a lower power level. This is a number that’s referred to in the solar industry as sun hours, which again is how many standard test condition hours per day the solar array effectively receives. Be aware of this: some will confuse sun hours to say there’s really only a 4 hour window when the solar array produces all of its electricity. That’s not really the case. It’s more of an average from the morning into the evening that gets compressed. Sun hours are really just a mathematical tool that the industry has used in the past to calculate daily solar performance, before tools like Pvwatts were available. If I know that in March I have 4.8 sun hours, then I know my 1kW array would average 4.8 kilowatt hours a day. It’s not something you’re really ever going to use when just getting into the solar industry. You might still use it today when trying to calculate the performance of a solar thermal hot water system.
Another spreadsheet line item is module efficiency. The module efficiency is an indication of how dense the solar array is. It’s not necessarily indicating build quality or cost-effectiveness, as an efficient module can be made by a lower-end manufacturer and an inefficient module made by a higher end manufacturer. Although to some extent your most efficient solar panels are generally correlated with your higher end manufacturers.
Here we see short-circuit current Isc and open circuit voltage Voc, as compared to the maximum power current and voltage, which is a factor of not the maximum current or maximum voltage, but a happy medium between the two. Imp and Vmp are more realistic expectations of operating current and voltage, in layman’s terms. When you do sizing calculations for the National Electric Code, you typically start the calculation by using the short circuit current and the open circuit voltage, so that when operating your system is performing underneath its maximum allowed rating, such that your safety calculations will not be exceeded unless something is going terribly wrong. So if you are making design calculations and wonder, “Am I supposed to do my voltage and amperage calculations based off operating voltage and amperage as compared to open circuit voltage or short circuit?” Use the short-circuit current the open circuit voltage when performing your safety calculations.
PVWatts is a very useful tool. It will tell you how many sunny days and cloudy days you’re going to have. It’ll tell you your rooftop temperature, the surface temperature of the solar panel,and your hours of operation. As you can determine when the array is turning on and off, you can use that window for a shade analysis to determine how much clearing is needed to get all the possible energy out of the array. Or you can also download the hourly production and discount the production during shaded times. The hour-by-hour data is useful for calculating value of time variable rate structures where the cost of electricity is higher at certain times and lower at others.
When you choose between different module types, like a premium module type verses a standard module, PVWatts slightly adjusts the temperature coefficient of power as further detailed in the PVWatts documentation. Although these are minor variables as silicon is silicon so there’s only so much you can do with it. But PVwatts does allow for slight adjustments, such as selecting between a roof mount versus a ground mount, which will actually model the ground mount at a lower operating temperature with slightly increase airflow underneath. But, if you actually run the math you’ll see the performance differences are negligible between most of these variables.
PVWatts allows you to do a single-axis or double-axis tracking array, and of course it allows you to adjust tilt angle and azimuth or compass orientation to fit your particular roof. And then, it posits a discount factor that accounts for all remaining production loss variables (assuming no shade) at a default value of 14%. I recommend just keeping that “as is” until you are more experienced with your designs, at which you point you can calibrate your discount factor to match field performance of arrays you have already installed.
When we talk about inverter selection, we’ll discuss module level panel electronics like micro inverters and DC optimizers, that allow every single solar panel on the roof to operate independently of each other. That’s going to reduce some of the mismatch losses know that might take that 14% number and improve it up to 11%, actually.
Let’s take a look at other factors of system losses that comprise the 14% derate factor. The first one is dust. Where I live, we get a lot of pollen in the air. A lot. You can see it in the morning on your front windshield. Then again, you’re not going to be getting up on the rooftop and cleaning your solar panels! I might recommend doing that once every 10 years, perhaps more if you live alongside a dirt road, but not often in general. So, PVWatts is assuming that the surface of your modules will have some dirt or dust on them, and so they’re going to be modeling a 2% loss from dust. They’re also going to model a 3% loss from shading, but that really accounts for very long shadows in the morning and evening, or something similar in effect like minimum energy requirements for your system electronics to turn on and off in the morning and evening, rather than shading loss caused by nearby trees. In other words, PVWatts may show that at 6:00 a.m. your 1kW solar array is producing 6 watts of power, while in reality that’s not going to be enough power to turn on and start up the system. So pvwatts accounts for that by throwing on a 3% shading discount factor, which I’d say is a good number for an unshaded array. If you’re doing solar on a shaded rooftop, at that point you have to start doing a 3D model and running simulations that are in the realm of commercial design software. We’ll take a look at some of that later.
PVWatts assumes that there will be no snow on your solar array. Here on the Gulf of Mexico, snow is not an issue. In Wisconsin, you could easily have a foot of snow on their roof for the entire month of January. So that means for your performance estimate, you may need to go out and just zero out the month of January, if there’s going to be snow on the roof every year and your client is not going to do anything to clear the snow off.
Imagine solar panels on your roof are like a chain-link fence; the weakest link in the chain is going to be the one that fails. Well, it’s not necessarily the solar panel that fails, but there is a tolerance that the panels have, meaning that some on the roof will be weaker then their neighbors. When I open a solar panel spec sheet, it’ll be for a 290 watt panel but also panels that are 300, 310, and also a 320 watt panel. So within the same form factor of the panel, I’ll get four or five different wattage ratings, efficiencies, and voltage and amperage characteristics.
The reason for that is that solar is made either in a sheet or a cylinder and then is cut in individual cells, or the cylinder is sliced into individual cells, and the cells that are in the middle of the crystal are a little bit more pure than the cells that are around the edges. So, these individual cells then get placed onto your solar panel, it creates a variance between the cells that result in a variance between the modules. Now, manufacturers test and sort the cells resulting in higher 320 watt panels and the lower efficiency cells comprise the 290 watt panels. All the cells are the same size, so all the panels are the same size, but out of the same production run,you get a higher efficiency module that might be sold to someone in New Jersey then you get a lower efficiency panel and that might be sold to someone in Mississippi, albeit at a lower price point.
Even within that, a 290 panel might actually be ratef for just under 300 watts. So, I open up a pallet of 290 watt panels and know all of them will produce at least 290 watts at standard test condition. The tolerances today are positive but used to be +/- 3% which is why pvwatts uses the 3% mismatch figure,which results in a discount factor that is too conservative. With most modules today, the mismatch is going to err on a production increase rather than a production decrease. But again, when presenting performance estimates your customers, it’s better to err on the side of conservatism.
At the same time, if I were using module level panel electronics like microinverters or DC optimizers (which we’ll talk about later), that mismatch would no longer exist at all. Weaker solar panels would no longer be dragging down the stronger solar panels. If I’m using microinverters or DC optimizers, I’m capturing 3% more energy, so instead of using a PV loss factor of 14% I might use a PV Watts loss factor of 11% and still feel like I’m still providing a conservative estimate.
Likewise, voltage drop depends on your particular project. if you’re up-sizing your cable you might get your voltage drop down to 1% instead of 2%. National Electric Code wants you to be around 3% currently, and 2% is generally what solar installers will design around. Actually, I’m often up-sizing my cable to take advantage of pre bundled cables, which often come with an undersized ground, so my designs have even less voltage drop.
Every time you splice a cable, square one section of cable is connected to another another such as when it lands on a breaker, or when one module plugs into its next door neighbor, that’s going to produce a little bit of resistance in the circuit. So, pvwatts is taking that into account.
There’s two types of degradation that aren’t fully considered by PVWatts, so while mismatch is being a little bit too conservative, leaving yourself some wiggle room in the discount factor well include other minor unaccounted for losses. The first type is called light induced degradation, and if that’s accounted for in the “nameplate rating” derate, then dust is not accounted for. Both light induced aggregation and dust are degradation factors that are measured in 1% or 2%. Basically this is saying that out of the box, your solar panels are going to degrade by ~2.5%, so while a 290 watt panel is going to test to 290 watts in the lab, when you unbox, leave it on the roof. It’s “nameplate rating” (we’re going to get into that with a warranty discussion and a chemistry discussion in just a minute) at year 1 will reduce at about .5% a year. So pvwatts assumes this is a new solar array but does not account for annual degradation with time. PVWatts is being a little bit too liberal in the fact that it’s giving you the Year 1 production value but Year 2, Year 3, Year 4, and so on will decline. Generally, I don’t worry about that too much, because I’ll assume the price of electricity will increase over inflation by ~.5% a year. That’s another thing you need to watch out for, some solar sales reps will grossly overstate how much the cost of electricity is supposed to increase. I’ve seen one company estimate as much as 7% year over year in a market where that number is under 2%. And even though the price of electricity may be going up, the value of your solar electricity may be flat, because that utility can monkey around with your value of solar through solar discriminatory rate adjustments.
Lastly, we get to “system availability”, and this is where I think PVWatts is, again, being a little conservative. They’re saying that the system is going to be unavailable for 3% of the year, which is 9 days. So they’re saying that for 9 days a year the solar array is going to be offline for maintenance and failure replacement EACH YEAR, and that’s just not what’s typical. If you do have a maintenance event or a failure of it , you may lose your production for a week while that replacement part is being bought, fixed, and installed, and then a couple of days for diagnosing it. So, 9 days might be typical for that, but if you’re having a maintenance event like that every single year you probably picked the wrong installer. Call me, and we can work something out. 🙂
And so, sum up all these factors and you get a very good, conservative estimate. Remember: it’s better to underestimate and over-deliver then oversell and under perform. Take the normal 14% degradation factor with PVWatts and that’s a solid number you can go to your client with. And I if you want to be a little bit more aggressive than that, just get rid of mismatch if you’re using DC optimizers or microinverters and adjust to 11%.
Let’s do an example. We’ll go to PVWatts and put in an address. I’m putting in a one kilowatt solar array with standard modules. By the way, the temperature coefficients used by pvwatts are conservative, and accounted for behind the scenes not being included in that 14% derate factor, so that’s another area where pvwatts is being conservative and that it will model more heat loss than what you should experience and your climate. Unless the temperature rises in the future but that’s another topic for another day. We select a roof mount, keep the system loss number the same, fixed tilt, and click this little info button to check your roof pitch. I do a lot of 5:12 rooftops at a 22 degree tilt angle. 12:12 is 45 degrees. I’ll just put on a 5:12 roof pitch to be a little more accurate and then select a due south roof orientation and click go.
We can see, living on the Gulf of Mexico, a breeze coming in keeping the modules a little bit cooler, with less humid air than in North Mississippi where we we have installs producing 1.4 kilowatt hours per watt per year, that we’re getting a little more production on the gulf coast at 1.5 kilowatt hours per watt per year. See that in January I’m producing 105 kilowatt hours for my 1000 watt array, whereas in July I’m producing 132. So that’s actually pretty stable production, reflecting a climate with sunny winters.
Then here, download the hourly information. Here’s my performance data for every single hour of the year. I get direct irradiance and diffuse irradiance, temperature, and wind speed. PVWatts is taking the array orientation and tilt angle and direct and diffuse irradiance and converting it into the amount of irradiance on the panel itself. I have my cell temperature, DC output and AC output.
If I want to ask what is the maximum value that my 1,000 watt solar array is outputting, I can see as my maximum output is 828 watts. As I have a 1,000 watt solar array it’s only giving me my 828 watts of maximum output, my inverter only needs to be 83% the size of my solar array. In other words, my solar array can be 20% larger than my inverter, and I won’t lose any energy even though my inverter is smaller than my nameplate array capacity.
Why is this? Because in the summertime, let’s let’s scroll to July, I look at my system output in the middle of the day and I’m getting 560 watts out of 1000. The next day 650 watts then 500 Watts. 650 Watts, etc If I put an 800 watt inverter on this system, I’m not getting up to my peak capacity in the summer at all. In the summertime I’m not hitting my peak capacity because of that temperature loss we talked about.
Instead, I hit the maximum 828 watts in April, with that lovely cool and sunny spring weather. For that matter in the summertime, the sun is more straight up in the sky so also a 20 degree tilt is better oriented for spring. Anyway, when choosing the relationship between my solar array and my inverter I am very comfortable undersizing my inverter by about 20%. By the way, if you do get up to the peak capacity, the inverter will simply leave that extra unconverted energy up on the array as voltage.
I’ve been referring to solar “panels” when what you call a solar panel the industry National Electric Code Book is a “module” not a panel. That’s because the term “panel” in Code is already reserved for your electric service panel or a roofing panel. We don’t want any confusion, so what we say is you have individual “cells” and the “cells” are combined to a “module”. Batteries are the same way. You have individual batteries “cells” and then the battery itself is called a “module” and they combine to form a solar “array”. We call the DC Circuits “strings” although I’m not quite sure why. I guess that’s what the cool kids say because code would refer to them as photovoltaic output circuits. We’re going to come back to that and our inverter selection later.
Solar modules are about the size of what one able-bodied construction worker can pick up and handle and set back down, which makes sense. A solar panel is 3.3 feet wide and about 5.3 feet tall. At the utility scale, they can be taller and heavier because you’re not handling them them on a roof.
Typically, at the residential level solar modules are 6 cells across and 10 cells tall and that’s called a 60 cell module. The utility scale is a 72 cell module because literally they’ve added two more rows of cells to the panel and made them a little bit taller and slightly more cost-effective. Note: it’s not that you can’t use 72 cell panels on your rooftop but I don’t recommend it, particularly on slanted roofs for the obvious reason of rooftop safety.
Solar modules are rated to withstand 1-inch hail at 50 MPH which makes them seem invincible. Solar modules are so strong they often give the installer a false impression of module strength. Twist a solar panel the wrong way or drop it on concrete and it’ll shatter the glass. The installer might be surprised because he just threw the solar panel on the back of the truck and nothing happened, but handled it wrong with a forklift and the module shattered. It’s a very specific strength. Let’s not forget the modules President Jimmy Carter put on the White House were in use until very recently, as in the last few years. So they do last. When you’re handling the solar panel on a rooftop there is some “give”. You can lean on them a little as you get more proficient at handling them (But don’t). Two tips are: first, try and put your weight on the frame rather than on the glass. You can spider-man across if you have to, but that is not a good idea. I’m trying to say solar modules more robust than weak.
The other tip is, when you’re installing is you want to make sure you’re not dragging this metal edge across the the glass surface of another module. I always remind my new guys to pick the module completely up. If you’re sliding it like when you’re unpacking a pallet, make sure at the very least you’re sliding the module across the frame of the one underneath it and not across the glass. If (when) you leave a scratch the warranty (which is typically 25 years although some of the higher-end modules have thirty year warranties) might come into question. They’re their long-term robust panels, so remember they will be fine. But don’t scratch them.
Now, module efficiency is how much power hits the surface v. how much power comes out. The backside efficiency ranges from 17% on the low end to 21-22 % on the high end of commonly used solar panels. Now, the thing about the higher end panels that are 22% efficient instead of 17% efficient is they can cost two to three times as much as the lower end efficiency panel. Typically, I don’t go for high end panels. It’s going to cost more, can be justified in certain locations. Generally those reasons are more affluent clients who simply want the best and also, they have a higher real estate resale value, with higher electric rates and limited rooftop space. So you’re paying for the array not to take up as much space but do the same amount of work. But if you have a more simple rooftop, lower efficiency panels usually result in a more cost-effective project. As far solar panel technology goes they’re pretty much all silicon panels, so there isn’t a ton of difference between low and and high end panels, compared to other system components like inverters or racking.
There used to be more discussion over what panel technology would be dominant in the past than there is today. You do get some non-silicon solar panels in the utility-scale market. There’s a big US manufacturer that makes them out of cadmium. This is not something you really want to put on your roof, because of environmental damage caused in the event your building burns down. Another global manufacturer makes CIGS panels which are: copper, indium, gallium, and selenium. Non-silicon panels are a little bit cheaper but they’re usually less space efficient, such that they’re really only used in large utility-scale projects in areas that are hot with cheap real estate.
This is my favorite upgrade. I don’t care about efficiency or brand names for solar. I am fine with solar panels that are generic. I’m much more particular about my inverter and my racking. But the one panel upgrade that I always recommend to my residential customers is to get all black panels. When you get low end all-black panels, there’s variations of all black and the grid lines show. (Although the further away you get from the array the more they vanish). If you treat your client to high-end all-black panels that are high efficiency and it’s just like a sheet of black glass covering the surface. It is gorgeous, if a little bit more architectural. I’m concerned about the aesthetics of my solar arrays, because the customer has to love it, and I have to love it as well. The only error that this installer made in this picture is that there’s a plumbing vent right here on the middle of the array and obviously they did the design for it to be one continuous row but then found the vent when they got up on the roof and I just stick the module off the end of the array instead. Maybe the client says, “Well, it’s on the back of the house so you can’t see it.” But the further away I get from the rooftop I can turn around and see it. We’re going to talk about how you can fix that in a minute. (No, it’s no big deal you just have to replumb that vent underneath the array.) I do like the look of one big rectangular black surface, like an infinity pool for the sun, as compared to the silver frame with a white back sheet. This has a black frame and a black back sheet and that eliminates all these little white spots that would otherwise show up on the array. Now there’s exceptions to that of course. If you have a silver metal roof then the silver frame solar panels can look quite nice too.
I’m very interested in is how to take a rooftop and build the roof out of solar panels rather than to use solar shingles for that reason. There’s a lot of solar installers who will grumble about Tesla “solar shingles” because they’re not available. I could be selling more installs but customers want to wait for “solar shingles”. Actually, solar shingles have been around for the last ten years. Inevitably they’re too expensive and the manufacturer goes out of business and stops making them and the solar shingle aesthetic really looks no better than having an all black array.
There are frameless solar panels that do not have the metal frame that goes around them, which is interesting. That frame is pretty useful because it gives you a support edge when you’re landing the module on the roof ,when you’re placing it, and when you’re lifting it. That frame really helps give the installer something to hold on to. If you’ve ever picked up a fin framed television you, you know what I’m talking about. You have to be more delicate with a frameless solar panel. The frameless panels shatter easily, but you might find a reason why you need them. I mean you can get a frameless solar array and then there’s no conductive edges. you might better fit them into a custom building frame.
I was looking at frameless panels for a utility scale project back in 2014 and the idea was that they could just be glued directly to the racking system and not only save a couple pennies per watt by not having the frame but also improve the installation time. The frameless solar panels really got set back by the Obama era import tariffs. I’ve only seen them installed in Australia rather than in the US.
Architects probably like this: these are called bi-facial solar panels. Just like the all-black panels took what was a white plastic backing and made it a black plastic backing, the bi-facial solar panels have a glass backing and that allows you to see through the panel. Now, I think it has real architectural purpose. You can make a stunning custom design for a covered walkway and better connects the public to the solar and all because you can walk underneath them and look up and actually see the cells above you. They’re more available than solar shingles and frameless solar panels, less available than all-black panels. They’rebeing considered more on utility scale projects simply to improve the density of the project, because they can collect sunlight from underneath the array as well as on top.
But collecting sun from the underside of the panel really is in a huge improvement and you can use PVWatts to explain why. Going back to our direct beam irradiance and diffuse irradiance, diffuse irradiance means there’s clouds in the sky. So here on January 1st it’s really diffuse irradiance. We have zero direct sunlight and indirect sunlight at high noon on January 1 and then on January 2, the same thing. January 3 same thing and here’s January 4th. A weather system had blown through and so what we see is that on this January 4th we got 964 watts of direct and 80 watts of diffuse and our solar panels are doing 728 Watts and then on January 1st we have no direct, all diffuse and we have 80 watts so 728 versus 80. So, let us assume that the underside of the panel is all diffuse light and so what we’re saying is maybe the bi-facial panels will boost the performance of the array by about 10% and they cost more, so if you’re trying to be cost effective you don’t really go for bi-acial panels. Typically they are a little more costly because the bottom is made from glass instead of plastic. But Jim pawns has been developing a clear plastic for solar panels that could really shake up the industry if it ever comes to market.
I would get bi-facial panels more for architectural reasons, like being incorporated into the building in beautiful ways, not necessarily in cost-effective ways. Think government building or community-based structures here, basically projects that would benefit from their visual beauty. At that point, it gets a little hard to find bi-facial panels because most of all are like this where the cells are really closely spaced together whereas I think that the ones with the cells spaced further apart are prettier and more logical because you want to let the sunlight through. But the further apart you space the cells, the more you’re buying a sheet of glass and not buying the solar panel, and so it becomes even less cost effective for residential, of course.
So, where do you go to buy solar panels?
Well, my advice is to go to online wholesalers and then sign up for their newsletter or whatever. They may say they have dealer pricing but if you’re not already a solar installer, they may not be so happy to give you dealer/installer pricing. So, put your email address in the newsletter because they’ll email pricing that is cheaper than what they’ll list online. So let’s say you want to do a solar project for yourself and it’s at least a pallet large, work the supply chain and you’ll be able to buy the solar panels at the same price that I do. It’s worth the time.
How does a solar panel work?
It’s confusing because it just sits on the roof and generates electricity, but how? Well, to start out with, a solar panel is a semiconductor and semiconductors conduct electricity under certain conditions and do not conduct electricity under other conditions. So, a photovoltaic semiconductor works like this: when sunlight hits this crystalline structure, these electrons here are in this rigid crystal but when sunlight hits that crystal it’s like a cue ball on a billiards table hitting the racked set of balls.
The cue ball is the photon, an energy particle of sunlight that causes the electrons to break loose out of that crystal and start bouncing around the cell or in this analogy a pool table and so at that point, it becomes an exercise of how to get all the electrons that are careening around the cell to go in the direction we want them to go? Conductive pathways are put into the cell and then it becomes a trick to get all the electrons to flow into these conductive pathways.
So to continue the analogy, take that pool table and pick one end up and all the balls fall into the side and go into the pocket. So the way manufacturers get all the electrons to fall into the holes is, they dope opposite sides of the silicon cell in two dissimilar elements which create a disparity between the top of the cell and the bottom of the cell that nudges the electrons towards one side. So the electrons bouncing through the cell have a tendency to be sucked up to the top of the cell and be repelled from the bottom, like a slanted pool table. And so, most of the electrons stack up on the top side of the cell where there’s no electrons, with not as many electrons on the bottom side of the cell.
The resulting difference creates a voltage potential just like you have on a battery with a positive and negative side. The solar cell, once that cue ball breaks the rack and keeping in mind you’re playing on a slanted table, now has a positive and negative side to the cell just like a battery, except it’s only on when those cue balls are hitting the surface of the cell and energizing the array. When you have sun you get a big positive and a big negative voltage. A little bit of sun, a little bit of voltage, no sun, no voltage and at that point it’s just a matter of connecting the positive and negative ends to a load and the electrons will flow from the positive end and back into the negative end. In this case the load is your house or more specifically, the load is the inverter that takes that DC circuit and converts those moving electrons to AC electricity.
Now when we were talking earlier about year one degradation, what happens is sometimes the electrons leap out of the cell and they don’t come all the way back and they will leak out of the inverter ground.
It used to be that with inverters, you would take the negative side and you would ground it, but nowadays we have floating inverters where neither the negative nor the positive are grounded and that reduces this what’s called PID degradation. Now with PID degradation, the electrons can also leak out of the frame so a frameless module has less degradation than a framed module although it’s not worth losing the frame over. The frame is very useful. Manufacturers have started to space the cells a little bit further away from the frame and they have less PID degradation.
That’s like when you hit the billiard ball and pollen of the ball falls off the table, so there’s build quality issues and system design issues that have been explored to reduce the PID degradation. This will happen mostly in hotter more humid climates, so it’s a particular concern on commercial flat roofs in the south where you get a lot of rain water just pooling right underneath the array.
The other degradation is called light-induced degradation and that is like the felt surface of a worn pool table and so the billiard balls careen around the table. Eventually, for purposes of this example, they etch little gutters into the felt. Well, then the electrons careening around the cell will fall into those gutters and lose a little bit of momentum. That’s why a brand new solar array is going to have a little bit more punch than an aging one, because those gutters have not been etched into the cell yet.
So in general you’re going a first year a step down in array performance. That’s warrantied by the way under most circumstances to be no more than 3% in the first then from that point on you have about .5% degradation per year. That .5% degradation is just from weather, such as humidity and water getting into the seal of the solar panel and then being vaporized and leaving little blotches that make the electrons not flow as well.
So now we’re getting into an advanced which are called snail trails. They’ve look like a slug just crawled across the solar array, but underneath the module glass. These are breaches in the cell where electrons have started to short-circuit within the cell themselves and the cell structure starts to rip apart and so these snail trails to some extent are inevitable at the end of a solar panel life, but can occur early in the module I fall resulting in almost no impact on a ray performance, depending on where they appear on the solar module. The electrons can still make it to the pathways unless they get so out of control that the electrons get trapped but even so the snail trails might only block one portion of the solar module, for an incomplete failure which might be further mitigated by inverter component selections discussed later. So this is something that will naturally occur on solar panels even when there is no measured solar panel failure.
This is a thin film snail trail with it which is a little bit more dramatic than a traditional silicon snail trail. Whereas the solar panel failure on the right got hit with a large tree branch in a heavy storm hit the array like a battering ram.
On the topic of water, with limited exceptions, you don’t want your solar panels to be flat. You want to give them at least a 10° tilt, and I even think more than that. Rainwater will clean the solar panel, but that effect is greater at steeper angles because at shallow angles the grime gets caught in the lip of the solar panel, and so on a perfectly flat array you’re going to get lots of dust and accumulation from rainwater evaporating and leaving behind even more grime on the solar panel.
On our module spec sheet, we see the module is rated for a snow load, wind load, and hail impact. The module frame is available in different thicknesses which can change how much wind load itc can take, so there are stronger solar module better suited for hurricane winds. We see the positive power tolerance that we now understand that comes from the manufacturing process. We see a maximum power decline of 0.7% a year and we expect it to be actually be around .5%.
We get the module dimensions and can see the size difference between 60 cell and 72 cell modules, the weight of the panel, even things like the length of the cable coming out the back of the junction box.
Even this data is useful as knowing these cables are going to be long enough so that the modules can be wired in either landscape or portrait is is vital to design. Managing these cables gets messy and so every installer has their favorite cable management techniques and when these cables are longer you can do even fancier things with your cable management than when they’re shorter. For example, we will often use metal clips to run these cables along the inside of the solar panel frame, with just a little bit of cable length peeking out where it’s supposed to connect to the next module on the circuit. that makes it easy for the installer to plug in the module, and eliminates the slack in the cable run that would otherwise droop down onto the roof surface. Most solar modules have cable whips long enough for the module to be mounted in either portrait or landscape.
Finally, on the module spec sheet we get to a warranty. There’s a 10 year product warranty and a 25 year performance warranty and that’s pretty common of your mid-range solar panels. High end panels often get a 25 year product AND a 25 year performance warranty. Well what’s the difference? What it really means is a 10 year warranty is on the build quality of the panel itself whereas the performance warranty is more like a limit on the maximum amount of LID & PID degradation over 25 years and so we see that there’s a year one warranty for 97%.
Well, now we understand why Year One light and potential induced degradation is greater than it is in subsequent years (because of those electrons etching those gutters on the pool felt) and we see that process reflected in year-over-year module performance warranties.
However, workmanship warranties should not be ignored either, as this is typically what you are purchasing when you go with a higher-end solar panel. Workmanship warranty address build quality, such as if the module frames started to come apart due to long exposure to the elements near the end of its life cycle. the standard workmanship warranty on a solar panel is only 10 years, what higher-end manufacturers greatly extending the length of this warranty to 25 years or even more.
Even the best warranty may not be a full model replacement, but rather compensation for lost electrical production. The manufacturer could go insolvent, although there are third-party holders of warranties. And even if you had a basic warranty, most warrantable module issues occur within the first few years of operation, and would reduce the performance of a module well below its performance warranty.
When a solar panel fails what is most common is that some little soldered connection from one cell to the next pops. What that’s going to do is it’s going to put a blockage in the circuit and your solar panels have bypass circuits so that if part of the panel becomes shaded, the rest of the panel remains on. That cannot necessarily be a huge benefit because just like one cell failing in the panel having impacts on the rest of the panel. One panel failing in the circuit can have impacts on the other panels in the circuit and so one part of the module being shaded will take the whole circuit down by one third and generally shading is going to occur on the whole panel and not just one little part of it .
But I guess what I’m trying to say by this is if you do have a little solder point pop and it takes out a third of the panel, well that’s going to trigger your warranty regardless of if it’s a stair-step warranty or a year over year warranty. And so whether it’s a 90% for 10 years than 80% three-year 25 or 97% year 1 96% year to 95% year 3, either way you’re going to be covered. If a solar panel arrives on site defective because it’s not just a half percent that’s going to be defective it’s going to be a third of the panel or so. The panel’s producing 2X the voltage of what it should be and that’s that’s going to be covered under your warranty whether it’s the most aggressive warranty or the most conservative warranty.
My general preferences to go bargain-hunting for solar panels, because I feel the difference in price is greater than the difference in value. However in hard-to-reach installations, as many rooftops can be, spending more money on a higher-end warranty could be worthwhile.
Further exploring the specification sheet,
I find this to be really useful: how many modules are in a pallet? When I do designs, I try designing an array that both pleasingly fits the rooftop, but also uses a full pallet of solar panels. Not all installers do this. Again, this is something I do to cut costs. So my customers usually get a 1 pallet or 2 pallet option or 3 pallet option and that might be a $25,000 project, $32,000 project, or a $38,000 project. Because I can go to a solar liquidator and buy a pallet but not an individual piece. Sometimes, I can go to a mainstream distributor buy by the panel rather than by the pallet, but it costs more, sometimes 50% more! And shipping by the pallet is easier.
A common audience question is, “Is the performance affected by workers stepping on the panels during installation?” The answer is yes. That’s the danger of the installer getting too comfortable with the solar panel ,as these failures can occur at the microscopic level so you’re going to have a solder point pop and you really can’t identify it with the naked eye. You could use a thermal imaging scanner to see a hot spot on the panel that would reflect open circuit voltage which would it reflect a popped soldered connection, but you wouldn’t be able to just look at the panel and see a wire that’s not connected in there.
On ground mounts the same thing can happen. You can get the ground mount built and then your workers might be leaning or sitting on the solar panel and think there’s nothing wrong because the panel itself is strong enough to withstand hail. But there could be a crack in the cell and sometimes the cracks in the cells don’t matter, like the snail trails earlier and the panel will keep working. Other times, it’ll result in the connection being completely destroyed and that results typically in one third of the voltage dropping out on the panel. Always try and put the weight on the actual frame and not on the surface of the glass!!
But what’s more likely to happen is that the installers on the ground get a little too lazy handling the panels, lean them up against the wall and a big gust of wind comes over and blows them over and they smack onto the ground and shatter the glass. Or, you’re unpacking them out of the box and then putting them onto a lift to lift them up onto the roof and you jerk the lift and the panels bounce up and down and how if they’re loosely stacked then yes, they can break. I’ve seen a bolt get underneath a panel frame and shattered the glass. Usually it’s going to be something like that, rather than putting weight on the surface itself, but it is somewhat common and this is another reason why I like ordering by the pallet, because they shipped to sight in a more organized fashion than when shipped loose. Loose solar panels might be stacked and shrink-wrapped and then the shipper may put additional boxes on top of the panel surface itself, resulting in damage during shipping.
So, the module spec sheet will give you a force load rating for the panels and likewise you’re racking manufacturer can give you different load ratings on the panel based on how the racking is used. If you need more strength, such as in a heavy snow or hurricane prone area, you may be able to add additional reinforcement, or find that a traditional rail mounted solar array is not even necessary for your roof. You may be interested in these figures if you want to mount the array a certain way to achieve a particular aesthetic.
One advanced racking type is called a shared rail racking system where normally you’re going to have two rails per module and they’re going to be about one foot in on each side. With a shared rail system you get rid of one of the rails, so that might mean fewer attachment points onto the rooftop and you save the cost of the rail, although the increase in labor cost is often greater than the material cost savings of a shared rail system. One of the more frustratingly difficult aspects of solar installation is cheaping perfectly straight rectangles and perfectly straight lines on top of a wavy roof.
Now when I look at shared rails I think they’re neat in the sense that normally there is an air gap between your modules on top of rack and so rainwater can fall into that gap and get underneath the array, but if you use a shared rail system at least for this gap, you can collect the water inside the rail and gutter it down to the bottom of the top. Worth enough forethought put into flashing and flashing tape, a watertight solar array can be created such that it could be integrated into the roof of a building, perhaps using bifacial solar panels to increase the aesthetic. I don’t know any installers who are doing this. I’m waiting for my first client who will let me do it with their garage rather than their house. Shared rail is harder to install. You have to be very, very precise for it to line up correctly.
Another unconventional racking system is a railless racking system, where you just put supports down onto the roof rather than the rail itself. Most residential installers are not fans of the railess system because you have to be more precise. Most rooftops are wavy. So rail is helpful because if you get the rail square, then the panels will go on square. But it so happens that on a standing seam metal roof top, railess is better structurally for load distribution. There aren’t real cost savings involved because you are buying and installing more attachment points to the roof. These attachments let you clips onto the roof instead of penetrating into a structural support member. Rail-less is something that you should do when your rooftop is unconventional, rather than as a standard design practice on a slanted roof.
So let’s talk about racking design. Now a very high-level solar design is how many rectangles can you fit into the larger polygon, (laughter) so that’s not too mathematically challenging, particularly for an engineer. There is some shade analysis although many rooftops are not shaded. I generally do shade analysis with 3D modeling using design software. It’s not always available in rural areas but Google Earth does it’s 3D buildings and trees and stuff, so you can, from the comfort of your desk, do a site evaluation with regard to shade.
The analog way to do it is to this device called a Solar Pathfinder and use it’s chart and a reflective dome. the x-axis has hours of the day and the y axis has months of the year, and where these shadows fall on to the chart will tell you what hours of the day and what months of the year you get shadows exactly where you’re standing on site. And so, what we can see is this in June and July, because of this tree, the solar array might normally come on at 7 a.m. although with spring forward that might actually be closer to 8 AM and then in December the solar array may not be turning on until 9 AM. Shade from this nearby house is not as big of a deal breaker as it seems. Generally, what you’re trying for sunlight at least from 9:00 AM to 3:00 PM as the best solar window everyday. The solar window is more narrow in winter and and wider in summer so maybe a better one would be from 9:00 to 3:00 in the winter and then from 8:00 to 4:00 in the summer. This is a fairly unshaded spot, but once the Sun gets into those branches there, it’ll turn off the array even though the tree will lose its leaves, the branches will still diffuse the sunlight.
You can also use good old-fashioned trigonometry for shade analysis. A few things: one is Microsoft has a map software search engine and it will often have more clearly defined pictures than what Google Maps or Google Earth will have.
Also, it’s always useful just to tell the client to take a few steps away from their house and turn around and take a picture of the roofline. Good on-site photography will tell you what’s going on.
I have a Google Earth trick I use frequently. If I’m looking at Google Earth and I see a tree, then I want to know the height of the tree. Commercial solar software has LIDAR data in it,but we can use trigonometry to start figuring things out like building or tree height.
You can find online charts that are called Sun Angle Azimuth charts. The US Naval Office puts out a really good one. So here we are on Google Earth. We put in our address and if I want to find out what the height of this of this tree is, I can use the Google Earth ruler tool to measure the length of this shadow which Google Earth is telling me is a 80-foot shadow.
The Google Earth ruler tool will also tell me the heading of the shadow or the azimuth of the shadow so it’s going to add a 160° with 180° being due south. Then Google Earth will also tell me,circled in red down there, the date the image was taken. So this shadow of this tree was taken on November 24th 2012, when the Sun was at 180° and the shadow was measured to be 80 feet long. Next, google Navy Sun angle azimuth chart. Here we are using the chart and I’m putting the date the picture was taken.
Let’s say Google Earth told us the photo was taken November 24 2012 in in Philadelphia, Pennsylvania and remember the bearing of the shadow was at 160°. We can now find the altitude of the sun as and so what the altitude and azimuth of the Sun.
The chart shows that when the sun was at a 160° bearing, it was 27° up in the sky. So although Google Earth won’t tell me what time of day the photo was taken, still I can determine the elevation of the Sun. So if I know the elevation of the Sun and the length of the shadow I can use trigonometry to calculate the height of the tree.
I can use the same technique to the height of the house. Don’t forget to subtract the height of the house from the height of the tree.
So if you don’t have access to fancy solar design software you can get an idea your tree heights and shading before getting out to the job site. Oregon has a good guide for converting shade objects into pvwatts loss… But if there is also of potential shade I think it’s best to use commercial design software for your shade model. Helioscope, for example, gives a free trial.
Then again accuracy is a relative term because trees grow, so you might not have to be as accurate in your shade analysis. What you need to do is overestimate your shade analysis because the trees will grow over time.
Wiind speed is greater at the corners and the edges of the roof rather than in the middle of the roof and some local fire code will require you to stay off the edges of the roof. There are ways around it, like forced air side vents in the Attic. But the bare-bones, most basic way to get the smoke out of a house on fire tis to get to the top of the roof and cut a hole in it. If there’s a solar array covering the roof, the concern is they can’t do that and so that’s been incorporated into residential code to say you have to stay 3’off the sides and 3’off the top of the roof at minimum.
To some extent, it is wise to leave an offet from the edge of the roof to the top and sides anyway. There is less wind load, and the extra space is welcome when servicing the array of every needed. If your roof overhangs the side of the exterior wall by three feet, that’s going to be a weaker area to penetrate anyway and so why not just leave it for the fireman so they can cut their hole in the roof? And remember it’s 3′ off from on both sides and the top because the fireman needs two ways to access the same roof plane. They don’t know what’s on the other side of the roof. They need to be able to go up one side, get across and then come back down a different way.
Fire code is a little more flexible than that, though. On a hip roof rather than a gable roof you can go all the way to one side without an offset, because on a hot roof, unlike a gable roof, you only have one hard edge with the other being a 45° step to the next roof surface rather than falling of entirely. But even in a roof valley, where you only need three feet offset rather than six, having a little extra spacecan be useful.
You don’t really want your solar array to be inaccessible in case there is a maintenance task you have to perform when you’re up there,. Now, hopefully there’s not going to be too much maintenance to do up there. If there are trees around your house and you get squirrels up there, yes there are squirrel guards. I’ll show you in a little bit but at the same time you can’t plan around everything. For one thing, code is encouraging the use of little electronic boxes behind every solar panel on the rooftop. That can help with de-energizing the array during the fire but also puts a failure point on the roof. Being able to get up on the roof and service that for one solar panel is not absolutely required but often useful. Otherwise, you would just start at the bottom, remove a few panels and then work your way up.
Commercial buildings have offset requirements too, and that makes a lot more sense as well, because there’s work done on a flat commercial rooftop. There are air conditioner units that need to be serviced that you can’t box in. Water drains that should be worked around rather than over. You need 4’ pathway requirements to get to your air auditioning units. Every 150’, you need a walkway. The standard walkway is 8’ but it can be 4’ as long as you’re putting little cutouts for the firemen every 20’. You have to be 6’ from the edges, rather than 3′ from the edges in residential, because you expect more walking around on this flat roof on a regular slanted roof.
Some safety codes seem a little excessive but there’s actually been a few solar fires that have burned down some commercial buildings. One big one was the Dietz and Watson 11 alarm fire in New Jersey in 2013 or the Bakersfield Target solar roof fire in 2009.
These fires occurred for a few reasons. What most solar installers know is that you have a combiner box where all the circuits come into before going down to the inverter and the old style inverter had a ground fault circuit in them that was grounded to negative, and what would happen under a very rare circumstance is one circuit on the rooftop ground faulting to ground would the ground fault detection system to fail. Then all of a sudden you get all of the circuits of the array feeding into that ground fault without a good means of didconnection and it start a fire that burns the building down. So most inverters are no longer negatively grounded and instead are floating, with the ground fault detection isolated from ground.
The ground fault detection blind spot fire in older commercial inverters allowed the fire to spread, but it began with the very hot metal conduit the rooftop home run cables were run through. The cables inside the conduit would expand at a different rate than the EMT conduit itself. This EMT conduit worked it’s way out of it’s fitting and cut into the cable. Thermal expansion is a consideration at 120′ or greater.
So these walkways are not just for access but also to ensure that you have broken up your metal so that thermal expansion doesn’t rip your homerun circuits apart.
If your commercial building has a slanted roof, it might qualify as a residential type roof, so we’d stay 3′ off the sides and 3′ off the edges. If using commercial clearance requirements we would have stayed 6′ off the sides (which is also OSHA friendly) and then also stay at least 4’ around the skylights, to produce an array like so. And since I had a little bit of space left over, I shift the whole array slightly over to center this panel right above the doorway to make the array look a little more square from the ground.
The general contractor should be able to figure out the starting point of the array. On particularly long or difficult array layouts, some installers will start in the middle of the roof and work outwards to ensure they are on center, but what’s more traditional is just to start on one side and work your way down in order to preserve that square-like appearance. Now one of the problems I ran into with this design is even though you stay away from the skylights all of a sudden these clearances around the skylights become walkways and so it’s not just you don’t have to just think about if the attachment through the roof is going to cause a leak. What is much more likely, because that solar installer is making that attachment and we’re assuming they’re good at their job they’re doing what they should be, it’s a brand new attachment generally what leaks is the pre-existing stuff on the roof. This is because a roof is not designed to be a jobsite.
When you get a team of workers tromping around on this rooftop around existing penetrations, that can be bad for the roof and so what I incorporated into my design was actually removing the skylights, and then we put more solar panels there and it was probably for the best that we did that. But, even walking around on a shingle roof can be bad for the roof.
The shingles protect the roof and when you walk on those shingles it gets grit off them, so that’s one reason to cover the whole roof, is that you’re then shielding the shingles with your solar panels instead of leaving them exposed to sunlight. What I try and do now is instead of assembling the whole thing on the rooftop, we’ll do is as much on the ground pre-assembly as possible. So we’ll build our rails and mount our electronics at attachment points all onto our rails as much as we can, depending on what lift equipment we have, we’ll do it all in one piece or into maybe three pieces but do as much on the ground pre-assembly as possible to reduce the time spent on the roof itself.
Electricity pricing varies by season. South-facing gives you the most production, but there’s a policy called net metering and the way that and we have this referenced towards the end of the program, one of the last slides well if you want to know what your net metering policy is there’s a website you can go to called desire usa.org desire without the first e in it so I’m putting it in the chat box so you can put in your state.
I chose Illinois, and here’s a list of all of Illinois’ green energy policies, as well as the federal. So you can find out information unlike the US tax credit stuff. I won’t spend too much time sorting through here, but I just type net metering. The federal government does not regulate the price of electricity. That’s left to the states. and so here we have Illinois’s net metering policy, and it applies to a variety of on-site generators up to two megawatts in size. There are some limits and net metering is a controversial policy. This has to do with what generation is pushed onto the grid by the solar array.
If you’re if you’re trying to offset 100% of your electric bill, but the solar array is only on for 1/3 of the day then de-facto 2/3 of the solar production is going to be pushed on to the grid. So, how you’re compensated for that 2/3 of the production becomes a controversial issue. You can view net metering as a consumer protection law that entitles the solar owner to better than default compensation. So with an Illinois net metering policy, they say if you are in a monopoly you’re credited at retail up through 12 months. If you are on a non monopoly, if you’re on a deregulated market where you can choose your electric provider. It’s credited at avoided cost at the end of the month. Now in Mississippi, the net metering policy is defined as no net metering, and we’re credited at avoided cost not at the end of the month but rather instantly.
So the instant we’re out-flowing onto the grid we get avoided cost as compensation whereas in this policy for Illinois, customers are credited at avoided cost at the end of the month. They subtract what you put onto the grid versus what you bought back, and any surplus is credited at avoided cost and so only a small fraction is credited at avoided cost and not the whole amount of outflow. If you’re in a monopoly they say because you’re in a monopoly, you get more rights than in a non monopoly and so you get credit at retail rate throughout the year until the end of the year.
This gets complicated because: what’s defined as the end of the year? They could define the end of the year as the end of spring or the end of fall instead of the end of December. So, even when you have a net metering policy that entitles you to outflow there’s more to the story. That’s why I’m love doing work with batteries and off-grid, because we don’t have to involve ourselves with that whole debate on whether it is right for the power company to charge you $.09 cents but only buy back at $.03 cents or to charge you at $.12 cents but only buy back at $.05. Someone’s going to be unhappy either way. Whether you have net metering or not I believe if you’re in a monopoly you should have net metering, and if you are not a monopoly then I can see why you would not have net metering.
Where it’s playing out is in Houston, Texas, where they don’t have any net metering laws but they have a deregulated grid. You get better deals for your outflow onto the grid than in Mississippi, where there is a monopoly but no net metering at all. There’s a clear example of deregulation benefiting the customer, so if you’re going to have a monopoly you should protect or entitle your customers to more rights than what they get the open competition. It’s more nuanced than that now but the reason why I put it out as South is best if you get retailed compensation for your outflow.
If you don’t get retailed compensation for your outflow, if you only get what the cost of the unrefined electricity is to the power plant, the cost of coal ,the cost of the natural gas, before it goes into the power plant. That’s the federal minimum buy back. The avoided cost of operation is what the federal government requires the power company to buy back your electricity. Then your net metering policy is a state policy that might entitle you to a higher compensation rate. If you don’t have a net metering policy, south-facing may not be the best deal. A west facing array or an east facing array may be preferable when you have higher loads. You’re getting up in the morning and running the air conditioner. At high noon there may be no one inside the house, and so an east facing array or a west facing array may produce less energy but will coincide better with your load and therefore produce less outflow.
Even though east or west may be producing 15% less, if the compensation for that outflow is 70% below retail, then that’s not going to do you any good to produce more, and so I would I would caution solar designers in states that have net metering. Most states do, and net metering has changed solar design. In a similar way that any subsidy impacts the unsubsidized business model, the 30% solar tax credit encourages solar installers to go for the top shelf. If I’m getting 30% off a Nissan or 30% off a Mercedes, I might go for the higher end product because I get that discount.
Similarly, if I’m getting paid retail price for all of my outflow, I’m going to design my array around what produces the most amount of energy, regardless of whether or not it’s in sync with the consumer load, for better or worse. Now, that makes net metering sound pretty bad. Actually, it might be one of the best energy policies we could have as a society, so there’s there’s more nuance in that argument, but what I would say isis don’t just think about your solar design has to be south-facing There’s more to the equation, particularly if your outflow rate is less than your inflow rate. For that matter, if you’re doing an off-grid design it might be that you need the most amount of energy possible, and so you’re just covering the whole roof. We made these arguments earlier on in the program but thinking about it even more unconventionally, if I’m doing an off-grid house, my most critical power may be in the winter.
I may just put the solar panels down the side of the building rather than up on the rooftop, or if I’m in New York City and I have electrical equipment covering the roof and I don’t have any surface area on my roof but I’m in a tall skyscraper, the side of the building might be the best fit. So, all you have to do to confirm non-traditional design ideas is just do a PVWatts calculation, and say I’m producing 1.5 kilowatt hours. Let me let me go with New York City, and so, here I am in New York City. I’m during the 1 kilowatt array at a 10° tilt angle on a commercial flat roof facing south.
My solar array produces 1.24 kilowatt hours per watt per year. If I go with a 90° tilt and run it down the side of the building, I’m producing 938 kilowatt hours a year. While that’s certainly it’s less but we’re talking it’s 25% less not 70% less. So for that matter, New York City electricity pricing being over double what it is in Mississippi, if the solar arrays were installed at the same price (which I admit is a stretch for New York City compared to Mississippi) but if they were installed at the same price, a solar array going down the side of the building in New York City would be more cost-effective than one installed in Mississippi.
Don’t be afraid of being creative with your array layouts! Let’s put some numbers to these economies of scale. Let’s just compare, doing three little small array sections on the south side of the building versus covering the whole roof with solar. So, we would be looking at a 4.5 kilowatt array. That’s a small solar array. There’s a minimum amount of money that you have to make to get up onto a roof and put solar panels on it. I might charge $2.75 per watt for a 13 kilowatt or $4.50 per watt for a 4.6 kilowatts and so if I do this small solar array at a substantially higher price, even though it produces more energy, it’s going to have a longer payback than if I do the larger array at a lower price. Even though it produces less energy per watt, the larger array is more cost-effective because of the discount that I’ve achieved due to economies of scale.
So the basic way that solar is attached to the roof is through this system of solar rail and brackets that are called L-feet. Ten years ago integrated L-feet with flashing did not exist, but today the standard way to do it is to get an L foot bracket and mount it to some flashing that slides underneath the shingle. Now it is not a perfect process. If you are doing new construction with shingles get the solar installer out there to put these flashing in as their shingling the roof because otherwise you have to get up underneath the shingle with a pry bar and pry out roofing nails.
Particularly if the roofer is trying to do a good quality job and puts in a lot of roofing nails rather than standard, it becomes a nightmare for the solar installer. There are above the shingle mounting systems that some installers are using but this flash pad, it doesn’t just go back to here, it goes all the way under this next course of shingles. That’s a good distance to get that water away from the penetration. So using flashed attachments as standard on a tile roof, you get a tile to replace your tile. They used to sell hooks that snake underneath the tile but that design has been phased out by the industry.
Instead they are using true tile replacements. There are manufacturers who specialize in making the tile replacements to fit the form factor of the tile you’re using. On the bottom right, we see a clip that clips to standing seam metal roofing. Out of all metal roofs, standing seam metal is my least favorite to work with. We’ll get into a reason why, but above the attachments you have the flushing, you got the elf foot, and then you have your rail. Regardless of the manufacturer you get clips that hook into this rail channel and they all do it a little bit differently.
I am partial to certain racking companies over others but the whole system is pretty much the same. You put a bolt into this channel, and then you have mid clips that space and clamp down the solar panel onto the rail between two panels and then you have end clips that go on the very end just for one panel and the clips have a grounding washer that grounds the panels to the rail. Then you ground the rail to your equipment ground conductor, and that gets run with your cables.
Pro installers are going to mount to every single rafter as they go across the roof and so the way that they do that without making Swiss cheese out of your rooftop is that they stagger the attachments. So, here’s a rafter,here’s a rafter, and there’s a 4’ spacing between their attachments. Sometimes in the interior of the array where there’s less wind load, that spacing can be stretched out to 6’ instead of 4’, but this is a very good standard. You’re not going to make any mistakes with this layout if you keep your attachments 4’ on center and then stagger them so that you’re hitting every single rafter. Now, not all rafters are 24” on center. A 16” on-center rafter is common as well.
So during the site evaluation what I do is I get in there and I measure the rafter spacing and then in AutoCAD, I locate every single attachment position to make sure that if I have if you have two rows of solar panels and your 4’ on-center, it really doesn’t matter about the spacing of your rafters. Instead of going every other ,every other, you can go 1,2,3,1, and hit every rafter as you go across, so that you evenly distribute the load over the truss of the roof.
I like using rail that has a little bit wider of a channel, where you’re using a lock nut that goes into here, rather than a bolt. The reason is, I like using the wide channel rail for cable management. It keeps my cable up on my roof secured and protected, and it doesn’t drip down onto the roof.
So what I do is, I take my electronics and cables and I build my rail down on the ground. I throw my cable into the top channel and share it with tie wraps and then when it comes time to put the panel onto the rail, the module frame itself holds the cable into the rail.
You can use tie wraps, you also get cable clips, but they serve different purposes. The tie wraps can get messy. They look cheap. People are concerned about them failing over the long run, but the cable clips though the module cable can actually pull out of the clip so it’s not a perfect solution either. I look at the cable clips as being a helping hand to pre-position the cable but neither zip ties nor cable clips are perfect. What I think is perfect is when the modules get clamped onto the rail. If you have your cable inside e rail then it’s permanently secured by the edge of the module frame. There’s no way that thing is ever going to come out of there and so the sign of a good installer is able to look underneath the solar array and be able to see all the way through it.
Even though we have little electrical boxes and droopy cable whips at every single panel, the end result is you can look underneath the array and can hardly see. You see the little boxes and cables right here and that cable then is going right into this rail, so no squirrel is going to get up under there and start chewing things up as you would if you had a rat’s nest of cable underneath.
A couple of finer tips: now this is called an array skirt or squirrel guard. How good are they? I have my doubts. They do restrict air flow. You definitely would want to use them if you have nearby trees and squirrels on the rooftop. Otherwise I think through good racking selection you get most of it done.
These are interesting. These are called snow guards and what they’re there for is to catch the snow so that it doesn’t slide off the roof all at once. So in areas of heavy snow, that could be important to prevent avalanches that can rip the gutters off the side of your roof, depending on how good your gutter guy is.
If you’re mounting the purlins instead of rafters, it can be a little bit trickier to get them to line up correctly. You might end up going with a landscape configuration instead of a portrait configuration.
Running the rail up the roof rather than east-west is possible, but it’s not preferred. It’s a more difficult install because you get up on the roof and either using a $600 stud finder. Not a $50 stud finder! It’s still tricky. You need a flat piece of cardboard and you use your stud finder through the cardboard, and then through the roof, rather than on the textured surface of the roof. The old-fashioned way is to take a rubber mallet and bang the roof and feel where the rafters are with the aid of someone inside the attic measuring it out. So what I do is I’ll go across the roof and I’ll bang the roof and I’ll take a whiteout marker and I’ll just dab the roof wherever I think there’s a rafter.
I’ll start at the top and do it in three different spots: at the top, in the middle, and at the bottom. Then look at my spots and see if from one road to the next if they’re lined up or not and once you get going with it it falls into place. So once you’ve identified your rafters, you put a chalk line to keep them straight, and drill a pilot hole. Once you drill your pilot hole, you’re going to know if you hit the rafter or not. If you missed it you can stick a wire down there give it a twist and see how close you are and then hopefully you don’t miss too often. But, that’s what the flushing is there for. So you call up and your guy misses and then you’re flush around it.
Now the reason why I don’t like standing seam is because it becomes a weakness of the standing scene panel itself because instead of lag screwing into the rafter, you’re now clipping onto the seam and relying on the strength of the standing seam panel. It is then attached to the roof which is nowhere near as strong as a lag screw into the rafter. The advice from the standing seam manufacturers is to clip to every single standing seam to get the best load distribution across the standing seam roof. Most installers don’t want to do that because you’re spending more money on your clipping system.
There’s little advantage to a railless system. It is harder to install than a rail based system and you’re not saving any money because you’re buying so many more attachment points and you’re covering up the roof anyway. I know standing seam is considered to be higher end than rolled metal but rolled metal is actually better for solar because your lag screwing into and through the roof, which terrifies the homeowner. But as an installer, I can tell you that the standing seam metal roofing products have an industrial butyl tape that goes underneath. You can see it here.
That goes underneath and so lag screwing through that butyl tape, which is very well sealed, and it’s a very strong attachment that you can be confident in.
Here’s another thing that I wish I had known on this previous project. There’s now conduit supports for standing seam or metal roofing systems that can help you keep that conduit up off of the roof. So that’s worthy of note.