Solar Photovoltaic Panel Watt Ratings: How much power do you really get?

Solar PV (Photovoltaic) panels, in the industry referred to as modules, come with a label stating the wattage. This can be very confusing to the public, and is poorly understood even by some people within the industry.

A watt rating on a solar module is a measure of “watts peak”, that is, the maximum amount of instant energy that can be produced under STC (Standard Test Conditions) and it is this wattage that goes on the “nameplate” rating of the module.  STC is the expected performance when a module is receiving precisely 1,000 watts of insolation (sunlight) per square meter of module aperture (area exposed to the sun) at a specific module operating temperature ( 77F) and when the module is installed at a certain angle to the sun (incidence angle).

Rated power can vary widely from actual power as conditions change. Changing the insolation, incidence angle, or temperature raises or lowers the instant power that will be produced. Temperature is a good example of variance. Each panel manufacturer specifies the thermal values for their panel to state the expected degradation caused by increased operating temperature. This degradation is usually somewhere close to .5% of efficiency loss for each degree C of temperature rise above 25C (77F). As you might imagine, solar modules frequently operate at temperatures much higher than 77F.

PTC ratings are a more realistic projection of a modules real world performance than STC. PTC is short for PVUSA Test Conditions. It is not “Performance Test Conditions” as some say, and actually PVUSA stands for Photovoltaics for Utility Systems Applications. What a mouthful.

It is possible when comparing two modules, that the module with a higher STC rating could also be the one to have a lower PTC rating. The CEC (California Energy Commission) uses PTC. You can look at their approved list of modules and see the difference between PTC and STC ratings. http://www.gosolarcalifornia.org/equipment/pv_modules.php For example, a Suntech Power STP230-20 is rated at 225 watts (STC), however the PTC rating is only 208 watts which may be closer to what you could actually get most of the time.

It should be noted that in cold weather, it is also possible for a module to produce more power than its nameplate rating. This variance in power production is what makes inverter selection and string sizing so important, because the modules best and worst case performance limits must fall within the inverters maximum and minimum operating range.

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