The Energy Grid

STC and PTC Ratings Solar



Some Sample Ratings

Brand Model STC (W) PTC (W)
SunPower Maxeon 3 (400W) 400 370
LG Solar NeON 2 (355W) 355 325
LG Solar NeON R (370W) 370 340
Panasonic HIT N340 (340W) 340 315
Q CELLS Q.PEAK DUO-G6+ 340 (340W) 340 310
Canadian Solar HiKu CS3L-350 (350W) 350 320
JinkoSolar Eagle 60M (JKM320M-60H) (320W) 320 290

Last Update: 2021

Each solar panel model may have varying STC and PTC ratings. Check the manufacturer’s datasheet or the California Energy Commission’s database for the latest ratings.

When buying a solar electric system, ensure the dealer provides the actual PTC-rated kilowatt hours daily.

Some dealers only provide STC-rated kilowatt hours per day, which is unrealistic. Ensure you compare similar ratings and insist on the actual system PTC rating, including module and inverter efficiency.

STC is an acronym for “Factory Standard Test Conditions,” which is 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 25 degrees C—cell temperature. PTC is an acronym for “PV USA Test Conditions,” which were developed at the PV USA test site at the University of Davis, California. The PTC rating represents a more real-life condition of 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature is 10 meters above ground level, and wind speed is 1 meter per second.

The ambient temperature rating is more reliable as silicon solar cells operate around 20 degrees C. Cell voltage decreases with rising temperature in real-world conditions, resulting in lower module power output compared to controlled factory conditions at 77 degrees F (25 C).

Cell voltage drops about 0.08 volts per degree C in environments that exceed 25 degrees C. Thus, an STC rating of 17 volts can become a PTC rating of 15 or 16 volts. Using Ohm’s Law, volts times amps equals watts, which equals power, so a reduced voltage means reduced watts. 

The California Energy Commission (CEC) PTC ratings are crucial for assessing solar panel performance in real-world conditions.

While STC ratings are determined under ideal laboratory conditions (temperature of 25°C/77°F, solar irradiance of 1000 W/m², and air mass of 1.5), the PTC ratings are based on a more realistic set of conditions that better reflect the actual operating environment of solar panels. The PTC ratings are derived using the following conditions:

  1. Temperature: 20°C/68°F (lower than the STC temperature)
  2. Solar irradiance: 1000 W/m² (same as STC)
  3. Wind speed: 1 meter per second (to simulate heat dissipation)
  4. Air mass: 1.5 (same as STC)

The PTC ratings provide a more accurate measure of a solar panel’s energy output by accounting for the effects of temperature, wind speed, and other environmental factors. This is important because solar panels typically perform less efficiently at higher temperatures, and real-world conditions are often less than ideal.

When comparing solar panels, it’s essential to consider the PTC ratings and the manufacturer’s advertised STC ratings. The PTC ratings will give you a better idea of how a solar panel will likely perform in your specific geographic location and under real-world conditions.

In summary, the California Energy Commission PTC ratings for solar panels are a valuable tool for consumers to understand better a solar panel’s actual performance under real-world conditions. These ratings consider various environmental factors and provide a more accurate representation of solar panel efficiency than the more commonly advertised STC ratings.