Standard Test Conditions (STC)
The standardized laboratory conditions under which solar panels are rated: 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. Panel wattage ratings are always at STC.
Why It Matters
Real-world conditions rarely match STC — panels often operate at 40-65°C (not 25°C) and irradiance varies throughout the day. This is why actual output is lower than the nameplate rating.
Related Terms
Kilowatt-Peak (kWp)
The maximum output of a solar system under Standard Test Conditions (STC: 1000 W/m² irradiance, 25°C cell temperature). This is the nominal system size quoted by installers.
Temperature Coefficient
The rate at which a solar panel's power output changes with temperature, expressed as %/°C. A typical coefficient is -0.3% to -0.4% per °C — meaning output decreases as panels get hotter.
Module Efficiency
The percentage of sunlight that a solar panel converts into electricity. Modern panels range from 15-22% efficient.