What is the difference between Solar STC and Solar PTC?

What does STC stand for and what does PTC stand for?   What is the difference when evaluating the name plate rating of a solar panel?



STC stands for Standard Test Conditions. These are measured under lab conditions of 1000W per sq meter of “sunlight” with a standard spectrum etc. It is a nominal or name plate value. For instance, a Kyocera 180Watt panel is 180Watts (STC). An array made from 10 of these panels is considered 1,800Watts (STC). When talking about the array size, the STC number is always used. It is a handy way of comparing arrays to each other, etc.

PTC stands for PVUSA Test Condition. This is much closer to real world conditions. The PTC value is used by California to figure your rebate. A Kyocera 180W panel is 156W (PTC)

Some blogs and websites are defining PTC as “Performance Test Conditions” but this is simply wrong. I have also seen “Pacific Test Conditions” or some such thing. Don’t believe everything you read.

Oh by the way, PVUSA stands for Photovoltaics for Utility Systems Applications. To makes matters more complicated, it is originally called Photovoltaics for Utility Scale Applications. 

Our friends at Go Solar California tell us:

PTC refers to PVUSA Test Conditions, which were developed to test and compare PV systems as part of the PVUSA (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. PV manufacturers use Standard Test Conditions, or STC, to rate their PV products. 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. The PTC rating, which is lower than the STC rating, is generally recognized as a more realistic measure of PV output because the test conditions better reflect "real-world" solar and climatic conditions, compared to the STC rating. All ratings in the list are DC (direct current) watts.

Neither PTC nor STC account for all "real-world" losses. Actual solar systems will produce lower outputs due to soiling, shading, module mismatch, wire losses, inverter and transformer losses, shortfalls in actual nameplate ratings, panel degradation over time, and high-temperature losses for arrays mounted close to or integrated within a roofline. These loss factors can vary by season, geographic location, mounting technique, azimuth, and array tilt. Examples of estimated losses from varying factors can be found at: http://rredc.nrel.gov/solar/codes_algs/PVWATTS/system.html.

One more key thing to know is that the STC rating is measured at an instantaneous moment in time taken with a flash tester.  It is not a rating taken over time.   Here is a picture of Keith Emery from NREL who is the world's leading expert in PV flash testing.