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STC

Standard Test Conditions (STC) are standardized operating conditions used for testing PV modules. They are defined as:
1000 W/m² Irradiance (global when in outdoor conditions).
25°C Module temperature
AM 1.5 Spectrum—a standardized solar spectrum corresponding to a path through 1.5 times the "normal" atmosphere (air mass at sea level).

For CPV (concentrating modules), the reference irradiance is specified in terms of DNI value (direct component).

Prior to spring 2011, the standard was typically 800 or 850 W/m² (not always clearly defined).

The current convention has standardized it at 1000 W/m².