The feed in tariffs have been instrumental in driving the demand for solar pv installations forward and there are strict guidelines laid down by the Governments MCS accreditation scheme on how the annual output in kWh is calculated, the annual output is the figure which is then multiplied by the Feed in Tariff (typically 41.3p).
The formula for working out the annual output for a Solar PV installation:
Output of the system kWp (no. of panels x max output of each panel in watts) x Energy co-efficient (0.8) x Solar Radiation Factor x Over-shading Factor = Annual Output
The Solar Radiation and Over-shading factors vary depending on the orientation and pitch of the roof and if there is any shading on the roof, for South facing roof the Solar Radiation Factor is 1042 and if there is no shading the Over-shading would be 1. For further information contact EnergyMyWay.