Below is a guide to help you work out how much your Solar PV installation would produce every year.
The equation commonly used to calculate this is:
Predicted Generation (measured in kWh/year) = kWp x Shading factor x Irradiance x 0.8
The kWp is the maximum power that your Solar PV system is able to work at, based on it functioning under perfect conditions in the lab. This figure is usually between 1.5kWp and 4kWp and is determined by the number of panels you have and their respective power output.
Shading describes the proportion of panel which is not receiving full exposure to sunlight. 1.0 describes a system where no part is shaded, 0.8 when there is slight shading and 0.6 where there is moderate shading.
Irradiance is the amount of energy that the panel receives from the sun. The table below can be used to find the appropriate figure for your property.
0.8 is an overall multiplier which allows for any panel downtime, unforseen poor weather or other conditions which may affect the efficiency of your system.
An example calculation for a 3.5kWp system which is slightly shaded, on a property facing south, with a roof slope of 30° would be:
Predicted Generation (measured in kWh/year) = 3.5 x 0.8 x 1042 x 0.8 = 2334kWh/year
While this is useful to deduce the capabilities of a panel in general, it gives no idea as to how the effectiveness changes at different stages of the year. It is reckoned that a panel is five times more effective in Summer months though, due to the brighter sunlight and longer days.