Welcome to our solar training program. Solar power is very scalable and many lessons in residential material selection also apply to commercial projects and utility scale, so this is as good a starting point as any to increase your knowledge of the solar industry. 

Feel free to ask questions and use the chat widget to communicate. In the next two hours we’re going to review modules, inverters, and solar racking terminology as well as balance  of system material. 

We’ll also talk about supply chain, such as how installers purchase material for small projects. This class will stay away from design scope. There are other classes. We offer that cover that which you can ask about, but let’s get started with this.

Solar transitions our grid to site-generated clean power which has many values. The more knowledge you have up front the better of the result will be. 

Modules, Panels, Cells

Figure 2 Modules, Panels, Cells 

The solar industry calls these solar modules, instead of solar panels, and we will use the term interchangeably in class.

But technically we call them solar modules, as electric code already uses the term panel to describe other important things, and also the term module does mean something specifically. 

An electric service panel holds the breakers that distribute power to a home, including the solar array. A roofing panel can be a section of roof that a solar array sits on top of. So the term solar panel is ambiguous.

Figure 3 60 vs 72 cells 

The term module in code is used specifically to refer to a grouping of individual cells. You might have heard of a battery cell. A 12V battery might be a single circuit of six 2V cells. Similarly, a solar module or panel contains a grouping of individual cells.

Most residential panels are 60 cells and most utility scale panels are 72 cells.  Counting the cells here there are one two three four five six and then along the length there are twelve. Six times twelve is 72 so this is a 72 cell solar panel.

This solar module has a black backsheet instead of a white backsheet but a trained eye can still count the cells. We still have six cells across but this module is a little bit shorter than the 72 cell module with only 10 cells down.

That makes a  60 cell module and the the difference between a residential 60 cell panel compared to the utility-scale 72 cell panel is these extra two rows of cells, which give the module a little bit more height, weight, and voltage.

A larger panel means slightly more bang for your buck all the way throughout the whole project, but the problem with 72 cell panels is they’re a little too tall and heavy for one person to safely handle on a slanted rooftop.

So residential rooftop solar will have the 40 lb, 3.3’ by 5.3’ 60 cell solar panels typically, again because they are easier to manage on the roof.

On ground mounts, or flat commercial projects, the larger 72 cell panels have slight cost advantages. A few specialty module manufacturers might deviate from these standards as well. 72 cell modules are allowed on residential roofs, they’re just a little harder to manage.

So a solar panel is  about 3 and 1/3 feet by 5 and 1/3 feet. Maybe a little different but that’s an easy size to remember. It’s just small enough for one person to pick up and walk around with.

The 72 cell panels used in utility-scale projects are 20% larger, with 20% more cells. They weigh 20% more and have 20% more voltage too.   So a 72 cell panel can be over 6 feet tall and that gets a little hard to manage up on a roof. A 60 cell solar panel weighs 40 pounds and so a 72 cell panel weights 50 pounds. 

Mono vs. Poly Silicon

Figure 4 Poly vs Mono Silicon 

Silicon for solar panels is grown as a rectangular sheet, known as polysilicon. Or it is grown in a cylinder and sliced like a salami called monosilicon.

Whether they are cutting the rectangle of silicon or the cylinder of silicon, the interior of the silicon is more pure than the edge condition of the silicon crystal, and now manufacturers sort the more efficient cells onto higher efficiency panels efficient panels. Cell sorting is one of many reasons solar panel efficiency has increased dramatically over the years. 

Less efficient cells go on to the less efficient panels which is why a module specification sheet will have the specifications for multiple solar wattages, all which have the same dimension. The cell sorting creates all these different wattages out of the same production batch and they are priced differently as well.

Mono crystalline solar is slightly better than polycrystalline silicon, due to these slight differences in the manufacturing process. But they’re like siblings born in the same year, in that they are very similar yet not quite twins. 

Futuristic thin film solar panels, such as translucent solar panels do exist, as do solar shingles, but all that stuff is a very small section of the market. Silicon is the clear market leader right now at a couple bucks a pound. 

On a side note: if a solar panel weighs 40 pounds, and silicon costs $1 per pound, then there is approximately $40 worth of silicon in a $150 solar panel.

Module Efficiency 

Figure 5. Module Efficiency

Solar module efficiency is the percentage of energy that hits the surface of the panel versus how much electricity flows out the backside of it. An impressive 20% of natural sunlight can be converted to electricity at its standard test condition. That number continues to climb.

Today’s solar panels capture almost twice as much electricity as the technology fifteen years ago, solar panels which are still producing today. Scientists themselves have underestimated how much electricity can be squeezed out of annual improvements in solar module design.

For example, a recent advancement in silicon is called PERC, which makes the bottom of a solar cell more shiny. This additional reflection increases total solar panel efficiency by 1%.

But let’s get back to basics with the specification sheets for now.

Module Specification Sheet 

Figure 6. Module Specifications

Let’s take a closer look at module specifications. The module power rating  is how many watts of power the panel will output under certain lab test conditions. This information can be used for pencil and paper design but nowadays most design decisions are guided by  computer design software.

This solar panel is an older model so it has a slightly lower wattage than most solar panels being installed today. Most 60 cell solar modules today are around 350 watts, just under 10 amps and 40 volts. 

MP or MPP stands for maximum power point. Basically the maximum power point is the operating voltage and amperage, rather than the maximum voltage and amperage. The maximum amperage is the short circuit current, and the open circuit voltage is how high the voltage would be on the circuit if it were fully energized without being under any load at all, like a light switch turned off with voltage on one side of the switch, and metal connected to ground on the other, with nothing but voltage potential in between. 

The maximum voltage and amperage are used in system design, rather than the operating voltage or amperage. A solar array may be designed for 600V, but will operate at a lower voltage. 

All the figures given on a module specification sheet are listed for a standard lab test condition. Just like a car gets a mile per gallon rating, the solar panel gets a wattage rating at a standard test condition.

But solar performance changes hourly, as well as seasonally, as the sun changes position in the sky. Overcast days diffuse the sunlight, and only sometimes is this enough to turn the panels off. 

Or the numbers can jump around on a partly cloudy day. So the numbers on the spec sheet are just a starting point, and how might they compare with solar performance on a normal day?

The standard test condition amount of sunlight is 1000 watts per meter square. There’s a free public software called PVWatts and as an example I’m going to pull up Biloxi MS.

Figure 7. Analyzing Environmental Conditions  

This is a PVWatts example for a 1kW in Biloxi, MS. Like other coast lines, Biloxi gets better production than further inland because clouds tend to blow over rather than stay still.

Anyway, at the end of PVWatts, the hourly results provide an estimate of the actual amount of sunlight that will hit the surface of the solar panel. PVWatts takes into account array tilt and orientation, and for this 18 degree tilt, south facing array, the array irradiance is listed on the spec sheet as the lab condition of one thousand watts per meter square in Biloxi is equivalent to high on a cold spring day.

Compared to this winter day in PVWatts, the solar array only gets 360 watts per square meter of sunlight. That’s only 30 to 40 percent of the sunlight used in the lab test and so the solar array only produces thirty to forty percent of its rated capacity on this cloudy winter day. On a sunny day, still in winter, the array gets up to 640 watts per meter squared.  Moving into July, temperature begins to impact the array by 10-20%, even if it had full exposure to a thousand watts per meter squared sunlight. 

So calculating solar performance isn’t as simple as taking the solar array size, multiplying by hours of daylight, and getting the amount of energy the solar array will produce from it. The wattage rating on the module assumes 1000 watts per meter square, but it rarely gets that amount of sunlight out in the field. A software like PVWatts is needed to estimate solar production. It’s easy to use and covered in other classes.

Figure 8. Rooftop Temperature

Then there’s this other part of standard test condition called the module temperature and that’s done at a 25 degree Celsius module temperature. How does that compare to rooftop temperature? 25 degrees Ceclius is equivalent to 77 degrees Fahrenheit, so how hot does it get on the roof when it is 77 degrees out?

Again PVWatts comes in handy. It provides ambient temperature column and module temperature, and it shows how to convert ambient temperature into module temperature.

But standard test condition is done at module temperature, and so standard test condition is performed on the equivalent of a 60 degree day outside, which is not very typical weather for most of the year.

Temperature Impact

Figure 9. Temperature Coefficients

Temperature Coefficients

On the module spec sheet, there are temperature coefficients to show how voltage, amperage, and power are impacted by temperature. 

For example, if the rooftop is 50 degree Celcisu and standard test condition is 25 degrees Celsius,  multiply the difference (25 degrees) by 3.4 percent and that’s a 8-9% loss of voltage due to increased heat.

The temperature coefficient of amperage has a positive sign. But the temperature coefficient of voltage has a negative sign. Whereas voltage decreases with temperature,  amperage increases with temperature. That’s worth pausing to memorize, but if you are having

Problems understanding voltage and amperage, I recommend starting with a volt times an amp is a watt. In other words, power is the multiplication of voltage and amperage. Voltage and amperage are different things that make up power, and we will leave it at that for now.

Bringing us back to our temperature coefficient discussion, the temperature coefficient of power is a -0.4 percent so at 50C, there is a 25C difference with standard test condition.

A 25 degrees temperature differential times .4 correction factor  percent means 10% of the power is lost due of temperature alone. 

Sunlight, also known as irradiance or insolation, has a linearly proportionate effect on solar module power. If the air has twice as much sunlight in it, a solar panel will produce twice as much energy minus any temperature effects or starting energy. This is our open circuit voltage with all voltage and no amperage.

The short-circuit current is the opposite, with all current and minimal voltage. The bend in this curve here is the maximum operating power of the Solar panel, with the right level of voltage and amperage to produce the most power under normal operating conditions.

So the panel will operate at a little bit less voltage than its open circuit voltage, and a little less Amperage than its short circuit current. 

Load vs. Voltage

Adding any electrical load to voltage will  reduce system voltage,

Because electricity is not infinite. Too large a load on too small a battery will lower the battery voltage to the point where it can’t deliver any power. Too much load on a grid can lower the grid voltage too.

But normally, our very large grid provides us with stable voltage because our loads are small and the grid is large. The solar panel too has an ideal operating voltage and amperage, and in the past, this would lead to a difficult discussion about keeping solar panels away from shade. Maximum power point tracking will reappear in the inverter discussion. 

An Outdoor Product

Figure 10. Solar Module Features

Solar panels are designed to go outside. Manufacturers hail test them at 1” thick at 45 mph, similar or tougher than most roof material.Installed correctly, solar can strengthen and protect the roof. But solar isn’t bulletproof.

It is strong and durable enough to give a false sense of security. It’s a quality object, something you can throw in a truck and drive down the road. By itself it will not likely break or shatter. If you drop it flat on its face, it might even be okay.

It’s durable and designed to face the elements. The specification sheet also reveals physical load data such as snow load and wind rating. The panels are tested for hail impact. They might have a positive or a plus and minus power tolerance which has significantly improved over the years. 

All of these numbers are fairly similar throughout the industry. A particular manufacturer might have a patent on one unique ability, but the features are more often standard than special. 

Some manufacturers provide different module frame thickness to choose in hurricane zones. Sometimes this information can only be found in the manufacturer installation manual.

Module Clamp Zones

Figure 11. Clamp Zones

Here’s an example that illustrates various ways to install this particular manufacturer’s solar panel to a rack. It provides different wind load and snow loads based on how the where the module clips are installed relative to the module frame.

Also shown is a technique of adding an additional rail for more reinforcement, such as to meet hurricane wind load requirements. 

MC4 Connectors

Figure 12. MC4 Connectors

There’s two dozen mainstream solar manufacturers still in this industry so by and large module quality is very good. The type of frame, the color of the frame, the gauge of wires coming out the

back of the panel, all of that information can be useful to know. One of the notable industry standards is the MC4 connector coming out the back of the solar panel.

A common point of system failure is the field-made MC4 connector, which creates the home run circuit connection between the array and the inverter. Even when doing things correctly, it can be easy to confuse the male and female housings and metal connection tips. Furthermore, the connector itself should not be exposed to any direct rain (such as dripping down in between modules) or accumulating water. So attention must be paid with cable management in locating the connectors, in addition to making them. Finally, they click together with a nice snap.

Figure 13. Module Dimensions

Even the information like how much length comes off the back of the module for that MC4 connector plays a role in advanced solar design, such as how to route the cables underneath the array. This is the

stuff that like solar installers take pride in, having the extra slack tucked out of the way in a workmanlike manner, as to not attract nuisances like squirrels or debris. 

Since we are starting with material selection, a more advanced material selection note is to review the voltage rating on these MC4 connectors. Mc4 is the connector standard in the industry, and some field made connections meet up with the plugs that come installed on the back of the solar module. While these connectors are universal, some are not rated for 1000V commercial systems. 

The detail in a module spec sheet can assist racking planning. some manufacturers will hollow out this piece of metal in the module frame

To save a little money. The US manufacturer SolarWorld does this with some of their panels. On standard solar, the robust module frame allows the racking underneath to run in portrait or landscape orientation and so the solar panels can be designed in portrait or landscape orientation, but not with these hollowed out module frames.

Register Module Features