PV Performance Tests

Performance ratings of PV modules are measured under standard test conditions (STC) of 1,000 

W/m2 of sunlight and 25°C cell temperature. In practice, however, the intensity of sunlight is 

usually less than 1,000 W/m2, and the cell temperature is typically hotter than 25°C. 

Additionally, once PV systems are deployed, several factors can impact their expected 

production (electric energy generated), including solar resource, temperature, and degradation 

due to the age of the system. Although the solar resource is variable, most of the variability is 

predictable based on time of day, time of year, and the angle that sunlight hits the PV module 

surface. In fact, the solar resource would be perfectly predictable based on clear-sky models if 

not for clouds, which are not as predictable.