Making sense of solar panel specifications

Solar panel technical specs don’t have to be scary. Monocrystalline? Polycrystalline? Read this guide to better understand the choices that are available.

— by The Solar Nerd — About a 14 minute read.

If you’re going to spend thousands of dollars to put solar panels on your roof, it’s a very good idea to take a few minutes to review the product specifications of the panels that your installer recommends to make sure you understand what you’re buying.

When selecting a panel, there are important tradeoffs, especially when it comes to price and efficiency. It’s important to understand what these are so you can communicate your needs to your installer. They want you to walk away as a satisfied customer, so a good installer will be happy to have this conversation with you.

But reading solar panel specification sheets can be a little daunting. They’re full of technical jargon, electrical ratings, and many industry standards for fire, quality, environmental, and electrical performance. Fortunately, out of the sea of numbers and acronyms on a typical spec sheet, there’s only a few that a homeowner needs to zero in on so they know they are getting the product they expect.

Just a couple terms to clarify first:

A solar cell is a silicon wafer that is usually 125mm x 125mm or 156mm x 156mm in size.

A solar panel or solar module is commonly made up of 60, 72, or 96 solar cells wired together. The number of cells isn’t something that’s important to you, but the dimensions of the panel is. Most panels for the residential market are about 65 inches x 39 inches, give or take an inch or two. When comparing two panels, make sure to check that they are similar size, otherwise the differences in power output can be misleading.

Clear? Okay, let’s dive in.

Monocrystalline vs polycrystalline

Solar panels for the residential market fall into two types: monocrystalline and polycrystalline, often simply refered to as “mono” and “poly”.

(Thin film is a third type, but the technology is relatively new and evolving. It does have the potential to be less expensive than standard panels, as well as having the capability for interesting applications, such as photovoltaics embedded in walls. This is definitely a technology to watch over the next few years.)

Silicon for monocrystalline solar cells is cut from an ingot of silicon that is a single, unbroken crystal. This requires a very high purity silicon and is more difficult to manufacture than polycrystalline. Mono cells are more efficient, less affected by high temperatures, and perform better in low light compared to poly cells. They also have an even black color that many people prefer.

Polycrystalline silicon cells are cut from a block of silicon made of smaller grains of silicon, giving a poly solar cell a flakey appearance due to the grain boundaries, as well as a blue tinge. Compared to monocrystalline silicon, poly generally performs a little worse and is less efficient. The main benefit of poly cells is that they are less expensive due to less complex manufacturing.

ProsCons
Monocrystalline
  • High efficiency
  • Better performance under wider range of conditions
  • Usually has a more uniform dark appearance
  • More expensive
  • Slightly fewer products to choose from
Polycrystalline
  • Lower efficiency
  • Somewhat wider range of product choices
  • Less expensive
  • Performs worse under high temperatures and low light
  • Can have a blue iridescent color that some find less attractive

Solar panel efficiency

The efficiency of a solar cell refers to how much of the incoming sunlight is converted into electricity. High efficiency is desirable in a solar panel because it means that you can use fewer panels to achieve the same power output, which gives you the option to cover less of your roof, or generate more electricity with the same roof area.

The tradeoff for high efficiency is a higher sticker price for each panel. However, because you can install fewer panels to achieve the same wattage, you can potentially save money with lower labor costs and less racking hardware by using high efficiency panels. These panels can also save you money if you are using microinverters or power optimizers, which require one device per panel. If you are going this route, it’s a good idea to ask your installer to price out the overall system cost with both a high efficiency and low efficiency panel to find out which has a lower overall system cost.

Highest efficiency solar panels

You’ll find efficiency listed on the datasheet of a solar module as a percentage. This number has been climbing over the past decade, and in 2019 you can find several modules on the market with better than 20% efficiency. The panel with the highest nameplate efficiency at the moment is the SunPower X-Series, which boasts almost 23% efficiency. Lower efficiency panels start around 16%. That ends up being a pretty big difference: it would take only seven 23% efficient panels to do the job of ten panels with 16% efficiency.

Another excellent panel is the LG NeON R series of panels, which reach as high as 21% efficient.

These highest efficiency panels are monocrystalline, and sometimes bifacial, which is a type of cell that can absorb light from both the front and rear of the cell, allowing it to capture extra light that is reflected behind the panel.

Standard test conditions (STC)

However, manufacturers always advertise cell efficiency that is recorded under standard test conditions (STC) in a lab setting. These are idealized conditions that don’t really represent how the panels will perform in the real world. This is because heat causes solar cells to be less efficient, and since solar panels are a dark surface sitting in the sun, they can heat up significantly and produce less electricity.

(It’s for this reason that your home PV system will often perform the best in the spring, when the sun is higher in the sky but air temperatures are still cool.)

STC test conditions specify a solar cell temperature of 25°C and light intensity of 1,000 W/m² (watts per square meter).

To get a better idea of real-world performance, you need to look at the product datasheet and find performance numbers labelled as PTC, NOCT, or CEC.

Real world performance numbers

NOCT, PTC, and CEC are industry test standards that attempt to better represent how solar models will perform in the real world. It’s arguably more important for you to understand these ratings than the nameplate STC rating of your panels.

NOCT (Nominal Operating Cell Temperature) uses the following test conditions: 800 W/m² irradiance (the amount of incoming light), 20°C ambient air temperature, and a wind speed of 1 m/s (meter per second) with the panel mounted at a 45° angle.

It’s important to note that the NOCT test specifies air temperature rather than measuring the actual temperature of the solar cell like the STC test does. This is a key difference because a solar cell sitting in the sun can be dozens of degrees hotter than the air temperature. (Think of how hot the hood of a black car sitting in the sun can get.) And because cell efficiency decreases with temperature, the NOCT test more realistically models how a module sitting on your hot roof will perform. Standardizing the wind speed is also important because of the cooling effect of breezes.

PTC (PVUSA Test Calculation) and CEC (California Energy Commission Test Conditions) are similar tests, except that they both specify 1,000 W/m² irradiance instead of 800 W/m².

An important note is that the California Energy Commission rating requires third-party testing, not just data supplied by the manufacturer. This gives the consumer an important tool to ensure they are buying a panel that works as advertised. All panels listed on the CEC website have met this criteria. (Scroll to the bottom of this page for a link.)

Manufacturers may list either NOCT or PTC/CEC ratings, or sometimes both. NOCT is the rating you will see most commonly. You will also see slightly different labels applied, so you’ll need to look closely. Here are a couple examples.

Product datasheet for Sunpower X-Series panelsProduct datasheet for Sunpower X-Series panels

The example above is a snippet from the datasheet for the SunPower X-Series. In the first column, you can see that the first panel listed is a 360 watt panel. (Again, STC is the nameplate rating, and solar panel power is listed in watts.) The second column is the nominal power (Pnom) under NOCT test conditions. You can see that it is 288 watts, which is significantly lower (20% less) than the theoretical maximum. This is closer to what you would expect to see in day-to-day performance.

Here’s another example, this time for a ReneSola panel.

Product datasheet for a ReneSola panelProduct datasheet for a ReneSola panel

This datasheet lists the STC output as PMax (maximum power). The STC PMax for this panel is 250 W, and the NOCT rating is 185 W, or 74% of the theoretical maximum.

Most of the panels we’ve reviewed have a NOCT rating that is about 75% of the STC output, so this SunPower model performs a little better than average.

As you can see, it’s not hard to understand these numbers, but it’s critical that you are aware of them. One thing to keep in mind: because NOCT and PTC tests use different irradiance values, they can’t be compared with one another.

Standing up to the heat

We’ve mentioned above that solar cells become less efficient as the temperature increases. This is a property you’ll find listed on datasheets as the temperature coefficient of the maximium power (PMax). Again, it may be labelled slightly differently by manufacturers, so here’s some more examples.

Temperature coefficients for Sunpower X-Series panelsTemperature coefficients for Sunpower X-Series panels

This is the SunPower X-Series datasheet again. You can see that there are three different temperature-related specs. Voc refers to voltage, and Isc refers to short-circuit current. We’re interested in the last column, highlighted in yellow, that tells us how much the maximum power output drops for every one degree Celsius increase in temperature.

In the case of all these panels, the maximum output drops by 0.29% for every 1°C increase, which is why the number listed is negative. In this case, larger numbers (closer to zero) are better, because it means a more heat-tolerant module.

Here’s the same Renesola panel again:

Temperature coefficients for a Renesola panelTemperature coefficients for a Renesola panel

You can see that this panel doesn’t perform as well, losing 0.43% power output for every 1°C increase. If you remember, this Renesola panel had an NOCT rating that was 74% of the STC, while the SunPower panel maintained 80% of the maximum. So, it makes sense that the Renesola panel has a worse temperature coeffient of PMax. As you can see, it’s a useful check to have these two different measurements as a sanity check against one another.

Getting crushed by snow or blown away

If you live in a hurricane-prone area or the snowbelt, you’ll want to pay attention to the mechnical or static load ratings of your modules. There are two ratings: the front side rating, often called the snow load, and the back side rating, or the wind load.

If you’re wondering why the rear of the panel is the wind load side, picture your photovoltatic system mounted a few inches away from your roof. When a strong wind blows against your roof, the air deflects away and upward, hitting the rear surface area of your panels. A solar module is lightweight but has a large surface area, effectively turning it into a sail. This can place a large amount of force on the panel and frame, so these mechnical load tests help ensure that your system can stand up to the elements.

You might see these ratings described on a datasheet as mechnical load, static load, or wind and snow load. The figures are listed in Pascals, which is a unit of pressure. Higher numbers indicate a stronger panel. A typical snow load rating is 5400 Pa, and 2400 Pa is common for wind load, but you will often see higher numbers. Here are some examples:

Mechanical load rating for a Canadian Solar moduleMechanical load rating for a Canadian Solar module

This is a Canadian Solar panel that lists 6,000 Pa snow load and 4,000 wind load. This is a higher than average panel for both backside and frontside strength, so this would be a good choice if you live in an area with extreme weather.

Mechanical load rating for a JA Solar moduleMechanical load rating for a JA Solar module

On the other hand, this JA Solar module has a 2,400 Pa rating for both backside and frontside strength, which is weaker than the model from Canadian Solar. This should not necessarily deter you from choosing this panel, especially if you live in a mild climate, but if you do live in a snow zone and are choosing between two panels that otherwise have similar price and specifications, you should consider going with the stronger panel for added peace of mind.

American vs foreign made panels

The vast majority of solar modules are made in China. This makes sense because China is also the largest consumer of solar in the world, having about double the installed solar capacity of the United States, and is continuing to install solar at a breakneck pace.

This means that there are fewer choices if you want an American made panel, but it is certainly possible. The problem is that it isn’t easy for a consumer to find out where their panel is made, because it’s not usually listed on the product sheet.

Also, you can’t tell where a panel is made simply by checking where the company is headquartered. Just like how a Chevy Blazer is made in Mexico while a Toyota Highlander is made in Indiana, you’ll often find Chinese companies making panels in the US, and American companies operating factories in Southeast Asia.

For example, JinkoSolar is a Chinese company that operates a factory in Jacksonville, Florida, where they make their 72-cell mono-PERC module.

SunPower is a US-based company with a factory in Hillsboro, Oregon. However, SunPower also has Chinese manufacturing facilities where its P-Series modules are made .

Mission Solar is one exception, which is a company that is headquartered and builds its modules in San Antonio, Texas. Another is the Tesla/Panasonic partnership, which is starting to manufacture its solar roof product in Buffalo, New York.

Beyond that, you have to do a little research to find out where your panels come from – or simply ask your installer.

Here’s a list of some of the largest solar manufacturers in the world and their manufacturing locations as of 2019. US-based locations are highlighted in bold.

HeadquartersManufacturing
Canadian SolarCanadaCanada, China, Brazil and Vietnam
First SolarUSAPerrysburg, Ohio; Malaysia, Vietnam
Hanwha Q CELLSSouth KoreaDalton, Georgia (under construction); China, Malaysia and South Korea
JA SolarChinaChina, Malaysia, Vietnam
JinkoSolarChinaJacksonville, Florida; China, Malaysia, Portugul, South Africa
LG SolarSouth KoreaHuntsville, Alabama (under construction), South Korea
LONGi SolarChinaChina, India, Malaysia
SuntechChinaChina, Germany, Japan
Trina SolarChinaChina, Thailand, Vietnam
Yingli SolarChinaChina

It should also be noted that the country of origin does not necessarily indicate the quality of the panel. A more important thing to pay attention to is the duration of the warranty, and also the financial health of the manufacturer.

All about solar panel warranties

Last but not least, you want to pay attention to the manufacturer’s warranty. This is often specified in two parts: a warranty on materials and workmanship, and a warranty on power output.

Materials and workmanship covers defects in the materials and construction of the panels. While failures are rare, they do happen. One scenario is a failure of the seals in the frame that allows moisture to get inside. Once humidity is inside the panel it can cause corrosion. Or, in a cold weather environment, moisture leaking into a panel can damage the wiring or cause the layers inside the panel to separate due to repeated freeze-thaw cycles. This type of failure would be covered by the material and workmanship warranty.

The other warranty covers the power output. Solar cells are exposed to high temperatures and UV light, and this causes the panel to slowly degrade over time. Panel quality has gotten better in recent years, and a panel manufactured today might lose only 0.5% of its original power output every year.

Typical warranties you’ll find are 10 years on materials and workmanship, and a 25 year performance warranty of 80%. That is, the panel is warrantied to still generate 80% of its original power output after 25 years. Some manufacturers offer longer warranties.

Finally, you want to select a company that has a good likelihood of still being around in 25 years so that you actually have a company to file a warranty claim with if you need to. Unfortunately, solar is a fast moving industry with tight margins, and there have a few bankruptcies in the past, with other possible bankruptcies on the horizon.

For example, Yingli Solar has been having debt problems for years , and unfortunately SunPower has been running a significant net loss for the last four years.

For Chinese or privately held companies, it can be hard to find reliable financial reporting. This is easier for North American companies. For example, Canadian Solar has had positive net income for a few years , and First Solar has recently become profitable and is carrying relatively little debt.

Further reading

Buying Solar For Your Home: The Complete Guide

This article is part of a series to help the homeowner go solar.