kWp / Kilowatt peak

kWp means kilowatt peak, a solar PV power rating measured in kilowatts under standard test conditions. It is used for the installed peak capacity of a panel or array, so a 4 kWp array is being described by its rated test-condition output rather than by the power it will produce in ordinary UK weather. Standard test conditions use a defined irradiance, module temperature and reference spectrum, which lets different panels be compared on the same basis. The commonly cited STC values are 1,000 W/m² irradiance, 25°C module temperature and an AM1.5 reference spectrum. Actual solar output changes with sun angle, cloud, shade, panel temperature, orientation, inverter limits and losses, so kWp is not the same as expected kWh generation. Installation paperwork and calculators may combine kWp with local yield, orientation, inclination and shade factors to estimate annual output in kWh.