An Introduction to Solar

Solar is a crystal lattice – you have to remember your high school chemistry classes for this one! Whether its polycrystalline or monocrystalline, the electrons within a solar panel are stuck in this kind of glass-like molecular structure. The way solar works is photons from the sun hit that crystal and knock the electrons out of where they’re supposed to be – it’s very similar to how a cue ball disrupts billiards on a table. In our pool table analogy, the solar panels have these electron holes remaining which want to attract electrons, but a trick is employed to keep the liberated electrons. Almost like picking up the edge of a pool table, chemicals are used to convince all the electrons to collect on one side of the solar cell. The positive charge form where the electrons were combined with the negative charge to where the electrons collect is a primarily driver for the photovoltaic effect aka solar power. So long as photons from the sun are hitting the solar panel, those electrons will be bouncing around the inside of a solar cell, getting collected along the silver linings etched into the cell, and create electrical DC current.

In general, one watt of solar produces 1.4 kilowatt-hours a year, or one kilowatt of solar produces 1,400 kilowatt-hours a year. That’s a little bit more production in very sunny climates like the Southwest United States, but anywhere from say, Houston to Minneapolis, the various combinations of sunlight and temperature average out to around that 1kW to 1.4 MW ratio, at least for roof mount arrays. Keeping this ratio in mind is useful for solar mental math – an electric bill that shows 17,000 kilowatt-hours a year of usage can be quickly estimated to be offset by as a 12kW solar array for example.

This ratio can be adjusted to your local area by using the website PVWatts, funded by the government National Renewable Energy Labs, which provides this data by considering local data from nearby weather stations. Put in your address, and you get a month-by-month printout of how much energy the solar array produces. PVWatts provides a wealth of useful data beyond just energy production estimates. You can use it to explore inverter clipping, check sunny days versus cloudy days, ambient temperature, rooftop temperature, wind speed – all sorts of things that are commonly overlooked but super useful for system design. PVWatts is an industry standard, and commercial design software (with more sophisticated features) commonly uses the same dataset that PVWatts pulls its energy estimates from.

Fundamentally, solar design is about how many rectangles you can fit on a larger polygon – it’s not very difficult to design a solar array layout. Fit the right number of solar panels on the roof and plug that figure into PVWatts to get an accurate solar energy estimate. There are some design rules and rules of thumb, like keeping the solar array accessible, shade free, and good-looking. Inverter products can mitigate shade to some degree, so if the solar array is mostly sunny, aesthetic, and easy to access, you’re on your way to being a entry-level solar designer. PVWatts does a small amount of international site locations as well.

Today, solar design is undergoing an industry change. Previously there’s been this standard operating procedure of of covering the entire rooftop with solar, driven by net-metering policy. Net-metering in the USA has traditionally required the utility to buyback solar electricity from their customers at a price that is advantageous price to the customer, such that the largest projects are the most cost-effective. But net-metering is being rolled back in most of the United States, making it time to consider the value of smaller arrays.

small solar works with TOU rates + batteries

If the idea of doing smaller systems makes solar installers nervous (as it would mean less work and money) well, the good news is that batteries are even more expensive than solar! Solar projects are supported by industry professionals because they have healthy project budgets, and even if the solar scope of the project is shrinking, the addition of batteries to a solar project keeps the project budgets within industry norms.

Plus, customers want batteries. Without batteries, solar arrays cannot provide reliable backup power during outages. Also batteries make economic contributions to the project, and can create more cost-savings than the solar array itself. As long as net-metering policies are replaced by peak and off-peak rates where the price of electricity varies based on time of day, a small solar array and battery system can significantly reduce an electric bill and provide some backup power during an outage. The most economic system might not be large enough to run an entire home for any significant period of time, but some backup power is still a lot better than none at all.

In short, residential and commercial solar installers with access to time-of-day or time-of-use metering should think about the projects being battery first, with the solar scope being as an add-on accessory option. That option might only be a small solar array on the easiest accessible roof surface, such as the garage directly above the battery. That solar array might be much smaller than what the installer might have previously considered a viable project. But today the goal is not just solar; it’s solar plus batteries.

Keep in mind, it’s not about how much energy you produce; it’s also about how much that energy is worth.

California is still a great solar market, even with net metering rollbacks. It’s very sunny in California and electricity is still expensive. Many Californians experience regular power outages. The solar market hasn’t gone away in California just because net-metering policy changed, but the project pathway to the market has changed. California is no longer a solar-only market; it’s a batteries-first market.

In contract to sunny California, the northeast coast enjoys its seasonal weather. But the northeast also has high electricity pricing, and it is also enjoys a robust consumer-owned solar market. The price of electricity varies more across the country than the amount of sunlight, so it is fair to assume that the price of electricity is the primary reason consumers move forward with a solar purchase. Backup power plays a secondary role, and multi-day whole home backup isn’t always in the best interest of the customer. Payback is what sold solar power in net-metering markets, and it attracted a larger customer base than customers prioritizing backup power.

The problem with today’s solar market is that the economics have now become extremely complex. Solar payback calculations used to be possible on a paper napkin – you cost savings equaled how much energy you produced multiplied by that net-metered rate. If you generated 80% of your home energy, your electric bill would reduce around 80%, regardless of whether it was consumed on site or sold back to the grid. There’s a whole discussion to be had about the role of simplicity in residential rate structures, and the number one argument in favor of net-metering as being the ultimate renewable policy is that homeowners have traditionally enjoyed the right to simple, metered electricity plans. The right to simple, metered electricity is eroding on many fronts, but we don’t have time to talk about that in today’s class. It’s much more complicated to calculate payback in today’s market. Since this is an intro class, let’s set aside those economics for now and talk about solar design. We’ll return to economic discussion later in this class.

Fire code rules found in building code govern array layouts, even if not enforced in all areas. A rule of thumb for residential array layouts is staying at least three feet away from the top and sides of the building, but leaving even more space is useful. The idea is that firefighters need access to the roof to swing an axe through it, if the area below is filled up with smoke. This access space has other benefits. Installers will need to get back up on the roof after installation. Wind speed is increased at the roof edges as well. All of this is considered when planning rooftop attachment points discussed later.

Commercial buildings have their own sets of setbacks – it’s more than residential, with greater spaces from rooftop edges, as well as walkway requirements and even how to orient the solar panel to minimize failure during a wildfire. Importantly, solar arrays must be broken up every 150 feet, although I think stopping sooner is a little better. Don’t forget to put expansion points in the conduit runs breakouts are there to help inhibit the impact of thermal expansion on your conduit, which is a common cause of rooftop solar fires. Fire protection systems installed within the solar array do not easily detect this kind of failure. Again, we will have more on racking components later.

Other important site evaluation questions can be answered by detailed pictures of the electric service panel, as well as “far away” photographs where the electricity enters the building, and perhaps a picture of the utility meter. That data can be useful when considering interconnection technique, which we will need another class to discuss. My residential preference is for hybrid inverters with 200A passthrough, using 48V battery banks, the largest of which is the Sol-Ark 15k. This is because the inverter is simply installed between the utility meter and the 200A service panel, providing everything needed for a solar, battery, and whole home connection, along with an additional port for a generator, EV charger, AC solar, or extra sub-panel with some added value in smart control for whichever device is attached. That keeps the site simple to design, easy to install, and flexible to accommodate any peculiarities.

Regardless of the inverter type, it is important to know the number of solar panels which can fit on a circuit. The Sol-Ark 15k is a string inverter with multiple MPPT, meaning that circuits are traditionally wired in series to form high voltage DC circuits with some tolerance for shaded rooftops. String inverters are great for sunny roofs and can be very shade tolerant in the hands of an expert, but that has not always been the case. Back when string inverters only has a single shade tracker, Solaredge created a DC Optimizer that put shade tracking at the module level. This also boosted the system voltage, so that more modules could be landed on a single circuit before landing on the string inverter below. Enphase took a different approach, eliminating the inverter on the side of the building, and making a micro-inverter, which was also installed behind every single solar panel on the roof, but producing 120V AC the same as what is used in USA homes. Out of these three inverter types, even when you include the various battery components, Enphase solution is the most expensive, and string inverters with rapid-shutdown only systems at the module or string level remain the lowest cost technical solutions on the market, at least, until the value of the manufacturer is added into the calculation.

Rapid shutdown is the ability for firemen to push a button (which can be a lockable button) outside the building and shutdown the solar array voltage on the roof. It took over a decade for the listing to be published after it was first implemented in National Electric Code, and in that time, SolarEdge and Enphase nearly a full monopoly on the market. By putting their computers behind every solar panel on the roof, they could abide with rapid shutdown requirements without having a listing. While ten years late to the party, the new UL3741 racking-RSD co-listing allows for string-level disconnection, which not only is a lower cost solution to rapid shutdown, while simultaneously eliminating the need for all those computers on the roof. So I fully expect the market to shift toward hybrid inverters with string-level rapid shutdown in the years to come, such as solutions being explored by Solodeck and Midnite Solar.

String-sizing, or the ability to calculate the right solar circuit for the job, is performed by manufacturer provided string sizing tools. It’s important to learn the calculations for these various tools, whether it be Enphase, SolarEdge, or Sol-Ark. But if beginning solar design, its also helpful to learn these tools, as well as those found in commercial software like Aurora Solar. There are even tools that generate complete permit packages, like Lyra (formerly SolarDesignTool). Lastly, handmade design packages are also inexpensive. Arka360, Aurora Solar, and Enact are examples of industry design software which fits in a wide variety of design and permit services into their commercial offers.

By and large, if you are unfamiliar with solar module selection, just visit some distributor websites and you will many options. A popular option is an “all-black” module, and there is even a range in quality for that feature. Its important to buy from quality solar manufacturers, but it is not important to buy the most efficient solar panel on the market, unless you have net-metering or a small rooftop.

Solar racking design warrants its own class, but the short of it is there are solar racking products for every rooftop, even if it isn’t a good idea. Like inverter manufacturers, many racking manufacturers have design tools on their website. I admire IronRidge for their leadership in this, because their design tools provide significant data, such as load diagrams and easy spacing requirements for those roof attachment points. So long as you abide by those spacing requirements, staggeting the attachment points across the roof rafters as uniformly as possible, and giving the the corners extra reinforcement (as well as bottom rows of rail in areas of heavy snow), these systems are fairly straightforward to install. There’s even rail-less racking systems for rooftops where you don’t have a clear picture of what’s underneath.

It would be nice to explore these topics in a bit more detail, so perhaps a part II of this introduction to solar is in order. In the meantime, thanks for being a good audience member!

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Posted March 16, 2024 by John Cromer in category "Blog