PTC RATING
Level Of Importance #2
Why Is It Important ?
There are two different performance ratings that are associated with a solar panel. One is the STC rating or (Standard Test Conditions) and the other is the PTC rating or ( PVUSA Test Conditions). The STC rating is a standardized test that is performed by the factory and is sometimes referred to as (Factory Test Conditions) and is basically the same as the label that is placed by the factory on the back of the solar panel. So if the factory test results indicated that a solar panel produces approximately 200 watts, then the factory will place a 200 watt label on the back of the solar panel. STC are 1,000 Watts per square meter solar irradiance, 25 degrees C cell temperature, air mass equal to 1.5, and ASTM G173-03 standard spectrum.
PVUSA test conditions were developed to test and compare PV systems as part of the PVUSA or (Photovoltaics for Utility Scale Applications) project. PTC are 1,000 Watts per square meter solar irradiance, 20 degrees C air temperature, and wind speed of 1 meter per second at 10 meters above ground level.
The PTC rating, which is lower than the STC rating, is generally recognized as a more realistic measure of PV (solar panel) output because the test conditions better reflect "real-world" solar and climatic conditions, compared to the STC rating.
This all may sound rather complicated, so to simplify: The higher the PTC rating the better the real world performance. The great news is that a list has been prepared by the State of California which shows the PTC rating of most solar panels so you can easily do a side by side comparison.
Again the point to remember is "The higher the PTC rating the better the real world performance"