What is the difference between STC & PTC?
STC stands for Standard Test Conditions. The manufacturer of the panels tests them at STC, which usually is at lower solar irradiance and lower cells temperatures. However, in the field the panels experience high temperatures, like 50 degree Celsius because of solar radiation. PTC stands for PVUSA Test Conditions and simulates the actual operating conditions more realistically. It is based on 1000 Watt/m2 solar irradiance, 20 degree Celsius ambient temperature, and 1 meter/second wind speed. The efficiency of the panels depends on temperature, and thus the PTC ratings are generally less than STC, whereas the nominated power output of a panel is closer to the STC rating.
