Standard Test Conditions (STC) are used to determine the power output of solar panels. Under Standard Test Conditions, solar panels are tested at 25°C (77°F) and exposed to 1,000 watts per square meter (1 kW/m2) of solar irradiance when the air mass is at 1.5.
Just like EPA mileage estimates on cars allow you to do some comparative shopping, the output of different solar panels under standard test conditions allows you to compare panels before you buy them.
What are Standard Test Conditions (STC)?
The 3 standard test conditions for solar panels are:
- Cell temperature: 25°C (77°F)
- Solar irradiance: 1000W/m2 (1kW/m2)
- Air mass (AM): 1.5
The amount of power a solar panel outputs under these conditions becomes its maximum power rating (Pmax), also called its nameplate capacity. For example, if a solar panel outputs 100 watts at STC, it will be labeled as a 100 watt solar panel.
However, your solar panels will rarely, if ever, experience standard test conditions. So after I explain what each of these conditions is, I’ll discuss what you can do with a solar panel’s max power rating to help you estimate how your panels will perform under real-world conditions.
Cell Temperature
Condition: 25°C (77°F)
25°C refers to the temperature of the solar cells within a panel, not of the panel itself, which, due to coatings, framing, and other factors, can vary. Temperature matters because solar cells are able to convert the sun’s photons into electrons most efficiently when their temperature is between 15°C (59°F) and 35°C (95°F).
Solar Irradiance
Condition: 1000W/m2 (1kW/m2)
Solar irradiance refers to the amount of light energy falling on a given area, typically measured in watts per square meter. Don’t confuse it with solar insolation, even if the two terms are often used interchangeably.
Solar insolation is a measurement of the average solar irradiance an area receives over time, measured in insolation hours. If you know the difference between watts and watt-hours, solar irradiance measures watts, solar insolation measures watt-hours. Solar irradiance at 1 kW/m2 is also known as a peak sun hour.
Air Mass (AM)
Condition: AM 1.5
Air Mass measures the amount of atmosphere that sunlight passes through before it reaches your solar panels. The more atmosphere, the more sunlight that is refracted or absorbed by air and dust.
At sea level, when the sun is directly overhead, the Air Mass is 1. Solar panel testers use an Air Mass of 1.5, to reflect a more common angle of the sun.
Maximum Power (Pmax)
The result of a test under Standard Testing Conditions is a panel’s maximum power rating, often referred to as its nameplate capacity or nominal power and denoted as Pmax. Pmax is measured in watts.
For DIY solar applications, the most common Pmax ratings fall between 100 and 200 watts. The average residential solar panel may have a Pmax of between 275 and 400 watts.
Pmax vs Real-World Power Output
Solar panels rarely output their max power rating, though. As a rule of thumb, you can expect a solar panel to output 70-80% of its max power on a clear, sunny day. It’s typical to see a 100 watt solar panel output 70 to 80 watts in direct sunlight at midday, for instance. And, of course, the output will vary greatly based on factors like shading, cloud coverage, and time of day.
Note: If your solar panels aren’t outputting the amount of power you expect, check out our tutorial on how to test solar panels to see if they’re working properly.
STC is an industry standard, but real-world conditions will almost always be different, especially in terms of temperature, solar irradiance, and module design. Solar panels are rarely exposed to 1 kW/m2 of solar irradiance outside of the testing lab. And STC assumes a cell temperature of 25°C, but solar cells come in modules that, being dark, can rise to 65°C (150°F). Mounting materials and form factors can also affect heat transfer and energy absorption. That’s why you need a few other figures to better estimate how a panel will perform in the real world.
Nominal Operating Cell Temperature (NOCT)
A separate testing standard called Nominal Operating Cell Temperature (NOCT) is aimed at reflecting real-world conditions. Its specs use an air temperature (not cell temperature) of 20°C (68°F), a solar irradiance of 800 (not 1000) W/m2, a wind speed of 1 m/s (2.2 mph), and the back side of the panel open to the breeze. This last point is great for DIYers, since your panels are more likely to vary in placement than those of a residential solar installation, where the back side is mounted on a roof and receives less ventilation.
Like its name suggests, what NOCT measures is the temperature of PV cells when exposed to these conditions. NOCT is not a replacement for STC, since it doesn’t determine the cells’ maximum output. The results of an NOCT test will be a temperature listed on the spec sheet of a PV panel, such as “NOCT: 50°C”
In order for information about the temperature of cells under NOCT to be relevant, you need one more factor that is often listed right under NOCT on a spec sheet: temperature coefficient of Pmax.
Temperature Coefficient of Pmax
Solar panels become less efficient as they heat up. Here in Maine, I likely receive fewer sunlight hours than someone living in Florida. Sunlight also hits my panels less directly than those in the Sunshine State. But when it’s 95°F degrees in Florida and 65°F in Maine, my panels are likely more efficient at converting sunlight into electricity than the same panels in Florida.
Temperature coefficient of Pmax measures the rate at which a solar panel’s power output drops for every degree above 25°C. A cell with a temperature coefficient of Pmax of -0.5% means a solar panel’s power output will drop by one half of one percent for every degree above 25°C.
Note: Some solar panels also list temperature coefficients of Voc and Isc, which indicate how the panel’s open circuit voltage (Voc) and short circuit current (Isc) change in response to temperature.
How to Estimate Solar Panel Power Output
Now that we understand NOCT and temperature coefficient of Pmax, it’s time to do some simple math to estimate solar panel output under more realistic conditions. As an example, I’ll use a solar panel with a Pmax of 400 watts at STC, an NOCT of 50°C, and a temperature coefficient of Pmax of -0.5%.
1. Remember that STC assumes a cell temperature of 25°C, so first we have to subtract 25°C from our NOCT of 50°C to get a temperature difference of 25°C.
50°C - 25°C = 25°C
2. Multiply the panel’s temperature coefficient of Pmax (-0.5%) by 25 to get -12.5%. This is the percentage drop in power output due to the temperature difference above 25°C.
-0.5% × 25 = 12.5%
3. Multiply 12.5% by 400 watts to get 50 watts.
12.5% × 400W = 50W
4. Subtract 50 watts from 400 watts and to get 350 watts, which is the expected power output when the cell temperature is 50°C.
400W - 50W = 350W
Again, that’s cell temperature, not ambient temperature. If the ambient temperature is regularly 50°C (122°F) where you live, you have bigger problems to worry about.
PTC
California Law requires solar panels include something called PTC (PVUSA Test Conditions), which uses STC lighting conditions but a set of real-world standards different from NOCT to give you a real-world power output estimate. On a spec sheet, you might see something like “PTC Rating = 350W”, but not every panel includes a PTC rating.
Your Mileage May Vary
If only life was as easy as simple math. Alas, there are many more factors to consider in estimating the actual production of the solar panels you end up purchasing. Most important will be solar insolation over the course of the year. Shading, panel angle, ventilation, geographic location, dirt, dust, and other factors play a large role in the efficiency of your panels.
Fortunately, in terms of purchasing decisions, all those factors are external to the solar panels themselves. Knowing STC, NOCT, and temperature coefficient can help you compare apples to apples as you shop around for the right panels for your DIY project.
