This part of IEC 60904 describes procedures used to determine the degree of linearity of any photovoltaic device parameter with respect to a test parameter. It is primarily intended for use by calibration laboratories, module manufacturers and system designers.

Photovoltaic (PV) module and system performance evaluations, and performance translations from one set of temperature and irradiance conditions to another frequently rely on the use of linear equations (see IEC 60891 and IEC 61829). This standard lays down the linearity requirements and test methods to ensure that these linear equations will give satisfactory results. Indirectly, these requirements dictate the range of the temperature and irradiance variables over which the equations can be used.

The methods of measurement described in this standard apply to all PV devices and are intended to be carried out on a sample or on a comparable device of identical technology. They should be performed prior to all measurement and correction procedures that require a linear device. The methodology used in this standard is similar to that specified in IEC 60891 in which a linear (straight-line) function is fitted to a set of data points using a least-squares fit calculation routine. The variation of the data from this function is also calculated, and the definition of linearity is expressed as an allowable variation percentage.

A device is considered linear when it meets the requirements of 7.3.

General procedures for determining the degree of linearity for these and any other performance parameter are described in Clauses 5 and 6.