by ecoSolargy

3 Differences between STC and PTC Ratings You Need to Know

solar-testcell1You may have wondered, “Does my 250 watt solar panel actually produce 250 watts?”  Well the answer, technically, is yes. Your panel produces 250 watts, but only under specific conditions.  Solar companies use Standard Test Conditions (STC) to rate their PV products.  The California Energy Commission (CEC), however, rates each product using the Photovoltaic Utility Systems Applications (PVUSA) Test Conditions (PTC) which calculates the wattage a solar panel can produce under “real world conditions”.

The three main differences between STC and PTC ratings are their purpose, temperature, and setting.


STC ratings are considered the industry standards for testing a PV modules.  On the other hand, PTC tests were developed to test and compare modules for use in California’s rebate program.  One could say that STC ratings are for manufacturers and PTC ratings are for customers!


Not only does sunlight factor into PV efficiency, temperature affects it too.  When PV products are placed in the sun, the module operating temperature increases relative to the air temperature.  When the operating temperature increases, less power is generated because of the power conversion costs.  STC ratings do not account for this air temperature variable like PTC ratings do.  Additionally, at 25 degrees Celsius, the quick light pulse used in STC tests doesn’t heat the module.  In environments warmer than 25 degrees Celsius, cell voltage drops 0.08 volts per degree Celsius.  So a module with a STC rating of 17 volts often has a PTC rating of 15-16 volts.

Laboratory Setting

Standardized test conditions are necessary to compare technical products.  These juxtapositions, however, don’t reflect actual product performance.  When the EPA tests automobiles for MPG ratings, they don’t drive the same way we drive. Similarly, standardized ratings on solar modules can’t determine exactly how your installation will perform. Now, let’s put on our lab coats – this section is about to get technical.

Using the ASTM G173-03 standards, STC rates PV products at 1,000 watts per square meter of solar irradiance (light energy) and at an air mass (optical path length through atmosphere) equal to 1.5 to produce the “nameplate” rating.  The PTC rating, however, is a calculated value and is often lower than the STC rating because solar modules are tested under “real world” conditions.  They are tested under 1,000 watts per square meter of solar irradiance, with a wind speed of 1 meter per second at 10 meters above ground level.  PTC ratings reflect “real world” conditions better than STC ratings.

Unfortunately, neither the STC nor PTC ratings can account for all real world conditions.  Many variables including geography, mounting techniques, time of year, and even having dust on the panel can affect a modules’ performance.  Despite this, STC and PTC ratings are helpful when choosing the suitable module for your project.  Standardized testing of modules simplifies solar for all of us.  If you’re planning to install solar modules in a unique environment, first contact the manufacturer so they can determine the product best suited to your application.


This post was written by our intern, Jeralyn Biscocho. She is currently a 4th year at the
University of California, Irvine majoring in Earth System Sciences and
minoring in Global Sustainability.

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