STC and PTC Power Ratings for Solar Panels
Solar Panel Ratings
Selling solar panels is no different than selling any electrical device to a consumer. They need to be presented in the most positive light possible and as specifications are the only way consumers can compare solar panels, solar panel manufacturers often put their best foot forward. Solar panel power ratings are often the main criteria that consumers use to compare the abilities of different brands and quoting the most favorable one may be too tempting to avoid for the marketing departments of some solar panel makers. The two main ratings to look out for are the STC and PTC power ratings.
STC Power Ratings
STC stands for 'Standard Test Conditions' and quoting a solar panel's STC power rating will present the solar panels abilities in the best way possible. Of course, the standard test conditions are far from realistic with temperatures maintained at around 25.5 degrees Celsius which will keep any solar panel operating at optimal efficiency levels. Solar panels tend to decrease in efficiency with temperature increase. The problem is that temperatures and conditions in the real world are more challenging to solar panels than the conditions experienced during STC trials. There is a more realistic test that one should use, if possible, to calculate the likely performance of solar panels before making a buying decision. The problem is, not all solar panel manufactures use them.
PTC Power Ratings
Another more rigorous test is the PTC or PVUsa Testing Condition. PVUsa is the name of one of the agencies approved to carry out these types of tests and involves more in depth testing. If you can find a solar panel with its PTC power rating, use this over the STC power rating as it likely to a more accurate rating. In fact, using PTC over STC power ratings is not the only measure to be taken for properly comparing solar panel. Power Tolerance and Temperature coefficients are also important in the effective comparison of solar panel performance.
Power Tolerance and Temperature Coefficient
Power Tolerance is the figure that manufacturers publish regarding the expected actual power of a new solar panel out of the box. A power tolerance rating of +/- 3% denotes that the manufacturer expects a 180 watt solar panel to achieve an actual power output of between 175 and 185 watts from new. One must also consider that the power tolerance will be quoted based on STC power ratings and not PTC power ratings. The temperature coefficient represents the amount of power lost as temperature increases and crystalline silicon solar panels are more affected by temperature increase than 'Thin Film' or amorphous silicon panels. The temperature coefficient of power rating or TCoP, represents the power lost with each degree (Celsius) of temperature increase. This would mean that a solar panel with a TCoP of -.0.3% will lose 3% of power for every 10 degrees temperature increase. In fact, a TCoP of -0.3% is good and the top solar panels will normally operate with a TCoP of around -0.3%. Even a temperature coefficient of power rating in the region of -0.5%, is acceptable and worry more if this rating is not published on specification sheets at all.