Temperature Coefficient

Temperature coefficient is a specification that indicates how much a solar panel’s power output decreases for each degree Celsius above 25°C (77°F). It is expressed as a negative percentage per degree, such as −0.35%/°C.

Solar panels are rated at a cell temperature of 25°C under Standard Test Conditions, but on a hot summer day, rooftop panel temperatures can reach 50–70°C or higher. Every degree above 25°C reduces output by the temperature coefficient percentage. For a panel with a −0.35%/°C coefficient operating at 55°C, the output drops by about 10.5% (30°C × 0.35%). This is why solar panels in hot climates like Arizona, while receiving more sunlight, may produce less per peak sun hour than expected. The temperature coefficient varies by panel type: monocrystalline panels typically range from −0.30% to −0.40%/°C, while some premium panels achieve −0.25%/°C or better. Lower (closer to zero) temperature coefficients are better, especially in hot climates. Adequate airflow between panels and the roof surface also helps reduce operating temperatures and mitigate temperature-related losses.