Standard Test Conditions (STC)
The reference conditions for rating PV modules: 1,000 W/m² irradiance, 25 °C cell temperature and an AM1.5G solar spectrum.
STC are defined by IEC 60904 and IEC 61215 so that every manufacturer rates modules on the same basis. Modules are flash-tested in milliseconds so the cells do not heat up, and their current-voltage curve gives the nameplate values printed on the datasheet: Pmax, Voc, Isc, Vmp and Imp.
- Irradiance: 1,000 W/m², perpendicular to the module.
- Cell temperature: 25 °C.
- Spectrum: air mass 1.5 global (IEC 60904-3), corresponding to sunlight passing through 1.5 times the vertical thickness of the atmosphere, i.e. a solar zenith angle of about 48°.
STC rarely occur outdoors: when irradiance reaches 1,000 W/m², cells are typically 20–40 °C above ambient, so real output is 8–15% below the label. Operating-condition ratings such as NOCT or NMOT (800 W/m², 20 °C ambient, 1 m/s wind) and the PTC rating used in California give more realistic figures.
Manufacturers sort modules into power bins with a positive tolerance (for example 0/+5 W), and flash-test reports usually accompany utility shipments. Laboratory measurement uncertainty at STC is around ±1.5–3%. The kWp rating, performance ratio and peak sun hours all refer back to these conditions.
Related terms
All terms →- Kilowatt-peak (kWp)
The DC power rating of a PV module or array under Standard Test Conditions; the conventional unit of installed solar capacity.
- NOCT
Nominal Operating Cell Temperature: the temperature a PV module cells reach under 800 W/m², 20 °C ambient air and 1 m/s wind on an open rack.
- Temperature coefficient
The percentage change in a PV module power, voltage or current per degree Celsius of cell temperature away from 25 °C.
- Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).
- Performance ratio
Ratio of actual AC energy output to the theoretical output implied by plane-of-array irradiation and nameplate capacity; a location-independent quality metric for PV plants.