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 Celsius in air temperature, and have a 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.
Neither a PTC nor a STC rating can account for all "real-world" losses and situations. Actual solar systems will produce lower outputs due to soiling, age of the system, technical difficulties, and other criteria. These loss factors can vary by season, geographic location, mounting technique, azimuth, and array tilt. Taking this into consideration, it is crucial to know how well your system has been constructed and how it is being rated.
To give an example, on an average 10 kW system, taking into consideration several factors, the overall DC (or the STC) rate to AC (PTC) rate can provide you with anywhere from 55% savings, to 100.8% savings. Based on these numbers, an average of 4 hours of maximum sunlight per day, and a cost of twenty cents per watt of energy, you could be saving anywhere from $1606.00 to $2943.36. Though many get these ratings confused, it is apparent from this example how important it is to know the difference between them, especially when it could be saving you thousands of dollars.
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