Making sense of solar panel specifications

Solar panel technical specs don’t have to be scary. Read this guide to better understand what terms like STC, NOTC, and wind load mean.

Last updated: April 28, 2023

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

But reading solar panel specification sheets can be a little (or a lot!) confusing. They’re full of technical jargon, electrical ratings, and many acronyms for fire, quality, environmental, and electrical performance standards. Fortunately, out of the sea of numbers on a typical spec sheet, there’s really only a few that a typical homeowner needs to focus on. This article will try to help you decipher all of it so you can be more knowledgeable about what you’re buying.

Where can I find the specification sheet for my solar panel?

The best way to get this information is to ask your solar installer. Often the model numbers for solar panels are a long combination of letters and numbers, so it can be easy to be confused about whether you’re looking at the right spec sheet. If you’re not at that point of meeting with installers and you’re doing your own research, look on the manufacturer’s website.

Manufacturers often have many different models, and a model line may have panels in a range of different wattages. Because of this, the model numbers for a solar panel are often long, such as LG380Q1C-V5, which is a 380 watt NeON R panel from LG.

Once you know the panel you’re interested in, the datasheets are often available as a PDF under the products, support, or downloads section of a manufacturer’s website.

Solar panel specifications that a homeowner might be interested in

A solar panel specification sheet will have several tables or lists of numbers and acronyms. If you’re a homeowner, you can safely ignore most of them. Many of them are electrical and physical specifications that are of interest only to system designers. Listed below are the handful of specs that a homeowner might want to know about.

Power specifications

Power output
Efficiency (STC)
Efficiency (CEC, NOTC, or PTC)
Temperature Coefficient of P_MPP / Power temperature coefficient

Physical specifications

Length and width
Frontside (snow) and backside (wind) load
Color

Warranty

Power warranty (length and degradation rate)
Product warranty

Test standards

IEC 61215
IEC 61730

The power specifications - power output, efficiency, and temperature coefficient - are what homeowners will most likely be interested in because they relate to the amount of electricity you can expect the panel to generate. That’s what this article will primarily focus on.

Solar panel power output

When it comes to marketing, the key specification that manufacturers focus on is the power output. Measured in watts, the power output tells you how much electricity that the panel can generate. The highest output panels these days are in the range of 400 watts and higher.

If you add the power output of all the panels in a home solar system together, you’ll typically have a total power output that is several kilowatts (kW). If you’re not familiar, kilowatts is a measure of power at a point in time, while kilowatt-hours is how you measure the amount of electricity that a solar array generates over a period of time. You can read my article on kilowatts and kilowatt-hours to help you make sense of these concepts.

A key thing to know is that the advertised output is known as the nominal or nameplate output of the panel and does not reflect how much electricity you can expect it to actually generate in real conditions. (This is closely related to the STC efficiency rating described below.)

Another factor is that the inverter in your system will almost always limit the maximum output of your system. This is known as clipping losses and is a normal part of a good system design.

Solar panel efficiency

The efficiency of a solar cell indicates the fraction of solar energy that the panel can convert into electrical energy, and is listed as a percentage.

High efficiency is desirable in a solar panel because you can use fewer panels and less surface area of your roof to achieve the same power output as a less efficient panel.

The tradeoff for high efficiency is a higher sticker price (usually) for each panel. However, because you can install fewer panels to achieve the same wattage, you can potentially save money due to lower labor costs and reduction in racking hardware. High efficiency panels can also save you money if you are using microinverters or power optimizers, which require one device per panel. With fewer panels, you won’t need as many of these of these expensive devices.

If you’re not sure which way to go, ask your installer to price out systems with high efficiency and low efficiency panels to find out which has a lower overall system cost.

Highest efficiency solar panels

Efficiency keeps improving as new technologies make it to market, and the average home installation now uses panels with better than 20% efficiency. At the time of writing, the panel with the highest efficiency is the SunPower M-Series, which boasts a maximum of 22.8% efficiency.

Another top performing panel is the REC Alpha Pure-R, which is up to 22.3% efficient.

Standard test conditions (STC)

The efficiency rating that manufacturers advertise reflects the panel’s performance under standard test conditions (STC) in a lab setting. These are idealized conditions that often don’t represent how the panels will perform in the real world. This is because heat causes solar cells to be less efficient. Solar cells are dark, and they can warm up a lot while sitting in the sun. It’s for this reason that your home PV system will often perform the best in the spring, when skies are clear and air temperatures are relatively cool.

The STC test standard specifies a solar cell temperature of 25°C (77°F) and an irradiance (light intensity) of 1,000 W/m² (watts per square meter).

To get a better idea of real-world performance, look for 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 panels 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, 20°C ambient air temperature, and a wind speed of 1 m/s (meter per second) with the panel mounted at a 45° angle.

An important detail is that the NOCT test specifies air temperature rather than measuring the actual temperature of the solar cell as 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.) Because cell efficiency decreases with increasing temperature, the NOCT test more realistically reflects how a panel sitting on your hot rooftop 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) specify the same test setup as NOCT, except with 1,000 W/m² irradiance instead of 800 W/m².

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 often. You may also see slightly different labels applied, so you’ll need to look closely. Here are a couple examples.

Product 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 (P_nom) 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 panel

This datasheet lists the STC output as P_max (maximum power). The STC P_max 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.

One detail to again emphasize about these specs: because NOCT and PTC/CEC tests use different irradiance values, they can’t be compared with one another.

Solar panel performance in the heat (temperature coefficients)

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 maximum power (P_max). It may be labelled slightly differently by manufacturers, so here’s some more examples.

Temperature 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 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 coefficient of P_max.

One potentially confusing detail is that manufacturers may also list current temperature coefficients (I_sc) and voltage temperature coefficients (V_oc). These specifications are all related to a panel’s temperature performance, but describe it using different electrical properties. Because homeowners are most interested in power output, we’ve focused on power temperature coefficient.

You can read even more about temperature coefficient if you want to explore this topic further.

Physical specifications: dimensions, strength, and color

While power output is often the main focus, solar panels have a few physical characteristics that a homeowner might be interested in.

It can be useful to know the length and width because solar panels don’t have standard dimensions. If you have a cramped spot on your roof, finding a panel that is a few inches shorter in one dimension can help you squeeze in panels where a bigger panel wouldn’t fit.

Another reason is that two panels with the same wattage could be different sizes. For example, a 400 watt panel that is 22% efficient is going to be smaller than one that is only 19% efficient. In particular, 400+ watt panels are a recent trend. Some manufacturers might achieve this rating simply by making their panels larger rather using more efficient solar cells.

Two specifications that homeowners in regions with extreme weather events will want to pay attention to are the frontside and backside strength rating, which are also known as snow and wind load.

You might see these ratings described on a datasheet as mechanical load, static load, or wind and snow load. The figures are listed in Pascals, which is a unit of force. 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. The front of a panel, with its strong glass layer, usually has a higher rating than the rear, which may instead have a plastic backsheet.

Snow load is important if you live in a place that can get multiple feet of snow in one storm, as we do here in Buffalo. It can also be important if you live somewhere that hail is common.

If you’re wondering why the rear of the panel is the wind load side, panels that are mounted on a ground mount can experience a lot of force from the rear when the wind is blowing.

Here are some examples:

Mechanical 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 module

On the other hand, this JA Solar module only has a 2,400 Pa rating for both backside and frontside strength. 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.

Finally, the color of your panels can be important if they will be mounted in a location that is highly visible, such as facing the street. Most manufacturers offer all-black solar panels, which have a uniform dark color. This is in contrast to a conventional panel in which the individual cells make for a visible grid. Many people think that a solid black color for a solar array looks better, especially if you have a dark roof.

All about solar panel warranties

As a consumer, one of the most important things to pay attention to is 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 less than 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, including ones that include labor costs.

To learn even more about solar panel warranties, read our article on solar panel durability.

Test standards for solar panels

On any specification sheet, you will probably find a long list of test standards that the panel has met. There are electrical, environmental, and safety test standards, and tests that are established in different countries and by different standards bodies.

Not only that, but test standards can have different versions which are often indicated as a year.

Some test standards are optional, such as IEC 61701, which is a salt mist corrosion test that homeowners who live in coastal settings would be interested in.

At a minimum you should expect to see IEC 61215, which is a large suite of quality and stress tests for silicon solar panels, and IEC 61730, which is a suite of safety test standards for solar panels.

Other technical terms on a solar panel specification sheet

You’ll find quite a lot more information than this on a specification sheet. Most of these technical details aren’t very interesting to the average consumer, unless you’re working on a DIY project.

Open-circuit voltage (V_oc): This is the maximum voltage that the panel can output. It’s called open-circuit because it’s the voltage of the panel if it were disconnected from any load.
Short-circuit current (I_sc): This is the maximum current (in amps) that the panel can output, which occurs when the cell is short-circuited.
Voltage at maximum power point (V_mpp): The voltage of a solar panel will vary as the power generation changes with the sunlight. V_mpp is the voltage of the panel when it hits maximum power output.
Nominal power (P_max or P_nom): This is equivalent to the STC or nameplate power rating of the panel.
Voltage temperature Coefficient (V_oc) and Current temperature coefficient (I_sc): Similar to P_max described above, the voltage and current of the solar panel will also be affected by head.