This International standard describes procedures for on-site measurement of crystalline silicon photovoltaic (PV) array characteristics and for extrapolating these data to Standard Test Conditions (STC) or other selected temperatures and irradiance values. Measurements of PV array I-V characteristics under actual on-site conditions and their extrapolation to Acceptance Test Conditions (ATC) can provide (see annex A and QC 001002): - data on power rating - verification of installed array power performance relative to design specifications - detection of possible differences between on-site module characteristics and laboratory or factory measurements - detection of possible performance degradation of modules and arrays with respect to on-site initial data For a particular module on-site measurement extrapolated to Standard Test Conditions (STC) can be directly compared with results previously obtained in laboratory or factory for that module, provided that in both measurements the reference devices have the same spectral and spatial response as described in the relevant IEC 904. Data from on-site array measurements contain diode, cable and mismatch lossos. Therefore, they are not directly comparable to the sum of the respective module data. If a PV array is formed with sub-arrays of different tilt, orientation, technology or electrical configuration, the procedure described here will be applied to each unique PV sub-array.