METHODS: Validating Real-Time PV Output

Intermediate
PV module label & specs
Some PV module specs can be found right on the module; others may need to come from the manufacturer’s spec sheet.
Measuring cell temperature
Cell temperature affects PV voltage, and therefore power. Operating temperature is often higher than standard test conditions (STC).
Measuring irradiance
Irradiance must be measured on the same plane as the PV array, and at the same moment as the power measurement.
PV module label & specs
Measuring cell temperature
Measuring irradiance

You suspect that your PV system is underperforming, but aren’t sure why. Perhaps it’s the weather. Maybe you added new loads to your home or had a lot of electron-guzzling guests. Either way, you need a way to verify whether or not the problem is your system’s performance.

Solar professionals can perform a system performance check, but it’s also something that’s not too difficult to do on your own. You’ll need a way to measure the modules’ temperature—an infrared temperature gun (~$90) or temperature probes on a digital multimeter. You’ll also need an irradiance meter (starting at ~$125), the modules’ STC specs, your inverter’s efficiency, and a way to measure inverter power output (your inverter’s meter or remote monitor will work).

It’s important to take these measurements in good sunlight, to synchronize the array power output reading with the irradiance reading, and having a helper makes it easier. You’ll take the power output reading from the inverter, and apply a calculation correction to the module specifications to account for the actual on-site conditions. Once you have your readings, follow the example calculations below.

Example

This example assumes modules have a positive-only manufacturing tolerance and have been installed within the past year (i.e., practically no degradation). The array is clean, and thus no soiling derates are applied. Wires are sized for minimal voltage drop of less than 1.5%, so wiring losses will not be figured in.

•           Array STC power (from module specs):

250 W × 10 modules = 2,500 W

•           Irradiance (measured with irradiance meter): 950 W/m2

•           Module temperature coefficient for Pmax (from module specs) = 0.45% per °C

•           Module temperature (measured at array): 60°C

•           Inverter efficiency (from CEC, bit.ly/CECinverters, or inverter specs): 97%

•           Power output (measured at same time as irradiance and temperature): 1,988 W

Calculation steps:

1.         Correct for irradiance (STC is at 1,000 W/m2):

950 W/m2 ÷ 1,000 W/m2 = 0.95;

0.95 × 2,500 W = 2,375 W

2.         Correct for the module temperature (STC is 25°C) using the module’s temperature coefficient:

60°C - 25°C = 35°C difference;

35°C × 0.45% per °C = -15.75% = -0.1575;

2,500 W × -0.1575 = -394 W

At 60°C, the array should produce about 394 W less than its STC rating.

Put these two derates together and we get:

2,375 W - 394 W = 1,981 W

3. Account for the inverter’s efficiency to calculate what the inverter’s output power should be:

1,981 W × 0.97 = 1,922 W

4. Compare it to the actual power output: 1,988 W actual versus 1,922 W expected.

Looks like this system is performing well within spec! For more information on PV system performance factors, see “Maximizing PV Performance” on page 50.

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