The SAP calculator underestimates the performance by around 2-14% in coastal areas and in much of southern England.
The SAP calculation says that a 1kWp array will product 858 kWh at a favourable orientation.
Whilst this holds true in central and northern England (the central strip from Coventry up to the Scottish border), it turns out to be understated by upto 14% elsewhere.
The map below shows the annual insolation per square meter across the UK (kWh per m2) and the electricity that can be expected to be generatd by a 1 kWp array.
In Reading a 1kWp array is expected to generate around 875 kWh per year. This is 2% more than the SAP calculation.
In the South West a 1kWp array is expected to generate up to 975 kWh per year which is 14% more than the SAP calculation.