The kWp is a system rating based on the wattage of each panels multiplied by how many panels there are. Therefore 8 x 250W = 2kW. However the declared net capacity of a system is generally the maximum output of the inverter or the kWp multiplied by the efficiency of the inverter. The estimated output is calculated by multiplying the kWp x shading factor x irradiance x UK constant. The UK constant is 0.8 which makes the figures relevant for our latitude and the irradiance is based on the pitch and orientation of the roof. The shading factor is 1 if there is no shade and will decrease as shade increases. This means if the roof was South at 35 degrees with no shade, the calculation would be 2 x  1  x 1067 x 0.8 = 1707kWh. This figure is only an average for the entire UK as is not region specific. Therefore a system can perform better in areas that receive more sun than average and worse in areas that receive less sun than average.