*Solar irradiation information is available through a shared EU photovoltaic GIS database. This provides an estimate of the amount of power generated by a solar PV system at a given location. In South Wales that varies from about 870kWh to 1100kWh per annum for each kWp of solar PV installed, with variation largely depending on proximity to the coast. We’ve allowed for an average of 900kWh in the illustrations above. So the calculation for the 10.8kWp system is 10.8 x 900 = 9,720kWh; 70% of that is 6,825kWh; 6,825 x 20 years gives a total output of 136,500kWh. If you divide the typical cost of the system (inc. VAT) by the total output then £11,976/136,500 = £0.088, i.e. 8.8p/kWh.’