How is solar irradiance measured?

Solar irradiance metering equipment used during the assessment of PV modules must have a spectral response that matches that of the PV module or system under test.

There are two irradiance measurement methods defined and accepted by international standards covering the performance measurement of PV systems:

Pyranometer
An instrument for measuring the intensity of solar irradiance, normally used to measure global irradiance on a horizontal plane. Pyranometers are generally high precision, high cost instruments using thermal sensors in a glass dome.

PV Reference Cell
A small PV cell with a known current vs irradiance characteristic, constructed using the same cell technology as the PV system under test. If they are not constructed using the same cell technology, an estimate of any uncertainty can be made. Spectral mismatch is of particular concern with thin film modules. Reference cells commonly have temperature compensation to ensure that the measurement accuracy is not affected changes in temperature.

Instruments such as light meters, lux meters or devices using photo diode sensors may appear to offer a low cost solution for measuring irradiance however they do not have the same spectral response as a PV module; they do not compensate for temperature and are likely to introduce significant measurement errors if used for solar PV applications. They are not suitable for PV system performance testing and assessment.