Open circuit voltage calculation using temperature and irradiance

A module's voltage changes based on the environment the module is in. The common method for calculating system voltage relies on the modules voltage temperature factor. This factor is applied for the coldest temperature to the Open Circuit Voltage, as calculated at STC conditions of 1000 W/m2. However, many of these coldest conditions occur at very low light levels. While voltage is largely temperature dependent, it is not solely temperature dependent and irradiance has an impact particularly in low light conditions. This finding is substantiated by both manufacturers and field measurements from operational systems. By using publicly available, multiyear satellite derived data sets, designers are able to show realistic Voc's far lower than the traditional method allowing in some cases several extra modules per string without the risk of exceeding IEC/NEC voltage limitations. Longer strings equate to considerable savings in a project be it through reduced home run quantities, more modules per tracker/racking, reduced DC line losses and keeping voltage higher and in the inverter's power point window for later in the projects life.