A system’s power output can be calculated fairly accurately given the array size (rating at STC), module temperature, solar irradiance, inverter efficiency estimation, and a basic system derate value. For this example, assume the system has been installed within the past year and that there is no shading on the modules.
Array size = 4 KW (STC)
Irradiance at array tilt & orientation = 80% of full sun (800 W/m2)
Module temperature efficiency = 84% (-0.5% per degree over 25°C—57°C assumed for this example)
Inverter “CEC weighted” efficiency = 94%
System losses derate value = 85%
Estimated array output = 4,000 W x 0.80 x 0.84 x 0.94 x 0.85 = 2,148 W
This figure can then be compared with system metering or output measurements to make sure the system is operating as expected.