PTC (Manufacturers Nameplate Rating) VS STC (PVUSA Test Conditions)

 

When shopping for a solar electric system for your home or business, make sure that the dealer that you are speaking with is quoting you in actual PTC rated kilowatt hours per day. We have noticed several dealers that are listing only STC rated kilowatt hours per day which is not the rating that the CEC uses to determine rebate amounts. Make sure when comparison shopping that you are comparing apples to apples. Insist upon receiving an actual system PTC rating which includes the efficiency rating of both the modules and power conversion unit. Value Solar always provides you with both the STC rating and the PTC rating of each system we sell.

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, power conversion unit 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.

Cell voltage drops about 0.08 volts per degree C. in environments which exceed 25 degrees C. Thus, an STC rating of 17 volts can actually become a PTC rating of 15 or 16 volts. Using Ohm's Law, volts times amps is equal to watts which equals power, so a reduced voltage, means reduced watts.