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.
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.