Besides intrinsic properties, the performance of a photovoltaic module is also governed by external parameters - one of the most important of them being the cell temperature. For maximum power output of the module, we need lower operating temperatures but higher irradiance. The typical temperature coefficient of power is -0.5%/°C for mono- or poly-crystalline silicon cells.

The cell temperature of open-rack modules, however, is governed by several external factors such as ambient temperature, irradiance level, wind speed, wind direction, and tilt-angle of the module in the array. NOCT ( Nominal operating cell temperature ) is the cell temperature when irradiance is 800 W/m2 , ambient temperature is 20°C and wind speed is 1 m/s at a module tilt-angle 45^o.
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