Module temperature. The higher its cell temperature, the lower the PV module’s voltage will be. With lower voltage comes less output (power). Even if it’s 25°C (77°F) outside, in direct sun modules will heat up much higher than the ambient temperature. Systems utilizing ground- and pole-mounted systems, and ones on elevated racks on low-slope roofs benefit from air circulation; in full sun, they tend to be 20°C to 25°C above the ambient air temperature. Systems that are flush-mounted on pitched rooftops have much less air flowing under the modules and can be 35°C to 40°C above ambient air temperature.
We could measure the air temperature with a thermometer and estimate the module cell temperature from the temperature adders above, but this would not be very accurate. A much better way is to use an infrared thermometer on the back of the module, if possible, since the glass front can cause skewed readings (see “Tools of the Trade” sidebar). It’s a good idea to take the temperature in a couple of places throughout the array and average the results. If the array is easily accessible, temperature sensors will provide the most accurate readings. Just be sure the sensors are measuring the module cell temperature, and not the ambient air or module frame temperature.
In our example, even though the air temperature is a moderate 18°C (65°F), we measure the module cell temperature at 53°C (127°F). Next, we need to convert this value into a temperature factor to use in our final calculation. First, find the difference between the module cell temperature and STC (25°C), converting Fahrenheit to Celsius:
Temperature difference = 53°C - 25°C = 28°C
Next, calculate the derate factor for the power we expect to lose from the increased temperature (it is preferable to use the specific module’s temperature coefficient of power instead of the estimated -0.5% per °C, which is quite conservative):
Temperature derate factor = 1 + (-0.5% per °C difference × 28°C) = 0.86
Our module cell temperature is higher than at STC, and the temperature factor should be less than one since the overall array power will be lower than at STC. If the module cell temperature is less than 25°C, then we should expect a temperature factor greater than one.
Irradiance. The more sunlight (irradiance) hitting the modules, the more current they will produce. Irradiance is measured with a handheld pyranometer. It needs to be placed at the exact same tilt angle and orientation as the PV array, ideally lined up adjacent to a module.
Many things affect the irradiance levels at the PV array, including time of day, season, elevation, cloud cover, and sometimes even reflection from snow. Unlike module cell temperature, irradiance levels can change very rapidly. A cloud passing by the sun can cause levels to spike, drop, and then return to normal (edge-of-cloud effect) in less than a minute! It’s important to look for a window of time, on a clear sunny day, when you can take a couple of readings that you expect to be similar, and record the inverter’s output power between the irradiance readings. Even better, have a friend assist, with one person recording irradiance and another recording inverter output simultaneously.
For our example, an irradiance reading of 875 W per square meter is recorded at 3 p.m. Although the sky is clear, there is a slight haze in the air. This was taken at the same time as the inverter output reading of 925 W (noted above). Since 875 W per square meter is less than the 1,000 W per square meter rating at STC, we should be seeing less power than the STC ratings, and an irradiance factor of less than one. To calculate, we simply divide our reading by 1,000:
Irradiance Factor = 875 ÷ 1,000 = 0.875
While irradiance levels of less than 1,000 W per square meter are common in most of the United States, at higher elevations, and with snow on the ground, higher irradiance levels are possible, yielding an irradiance factor greater than one.