I have had a system installed that has a size of 2kWp but the declared net capacity shown on the MCS Certificate is 1.90 Kw and the estimated annual generation is shown as 1,650 kwh. Could you please explain to me the reasons for the difference between these figures?

December 20, 2012

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.

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