Standard Test Conditions (STC)

Standard Test Conditions (STC) define the specific laboratory environment under which all solar panel ratings are measured, enabling fair comparison across manufacturers. The three STC parameters are: 1,000 watts per square meter of irradiance, 25°C cell temperature, and an air mass (AM) 1.5 spectrum.

The 1,000 W/m² irradiance level approximates bright, direct sunlight on a clear day. The AM 1.5 spectrum simulates the spectral composition of sunlight after passing through 1.5 atmospheres of air — roughly the spectrum at mid-latitudes when the sun is at a 48° elevation angle.

The 25°C cell temperature is particularly notable because it rarely occurs in practice during sunny conditions. When a solar panel is producing meaningful power in direct sun, its cell temperature is typically 40-70°C, well above the STC baseline. This is why real-world output is almost always below the STC rating on hot days.

Some manufacturers also provide ratings at NOCT (Nominal Operating Cell Temperature) conditions — 800 W/m², 20°C air temperature, and 1 m/s wind speed — which are closer to real-world rooftop conditions. NOCT power ratings are typically 20-25% lower than STC ratings and give a more realistic picture of everyday performance.

When comparing panels from different manufacturers, always use STC ratings for apples-to-apples comparison. When estimating actual energy production, apply temperature derating and your location's actual irradiance data to the STC numbers.