All PV modules have information shown on the rating plate which relates to the module performance, including values for open circuit voltage (Vo/c), short circuit current (Is/c), voltage and current at the maximum power point (Vmpp and Impp) and the maximum power (Pmax). When PV modules are produced, manufacturers check the performance and determine the rating plate values by carrying out an I-V curve measurement.
The voltage and current produced by a PV module depend upon the irradiance level, the spectral content of the irradiance and module temperature. If the data on a PV module rating plate is to be meaningful, we need to know the conditions when the measurements were made. Manufacturers measure the rating plate values under standard test conditions (STC) with an irradiance of 1000W/m2, a module temperature of 25oC and using an irradiance source equivalent to an air mass AM=1.5.
If we want to compare performance data measured in the field directly with the rating plate values, the measurements must be converted to STC. The conversion requires measured values for the irradiance level and PV module temperature at the time of measurement.