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What is the difference between Solar STC and PTC?

Release:2017/06/12 Company News

STC stands for Standard Test Conditions. These are measured under lab conditions of 1000W per sq meter of “sunlight” with a standard spectrum,25°C cell temperature and AM1.5g spectrum according to EN 60904-3.

It is a nominal or name plate value. For instance, a NSP (Neo Solar Power) 260Watt panel is 260Watts (STC).  


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. These test conditions were normal operating cell temperature which is typically around 113°F (45°C), 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. PTC value is used by California to figure your rebate, for instance, a NSP (Neo Solar Power) 260Watt panel is 236.5Watts (PTC).  You can find PTC test results at GoSolarCalifornia.


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.



However, 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 etc.