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SAP Calculation

What is the SAP calculation?

The Sap calculation calculates the output of a solar PV system according to the Government’s Standard Assessment Procedure for Energy Rating of Dwellings (SAP 2009)

The calculation is as follows:

  1. Establish the installed peak output of the solar PV system (kWp)
    In simple terms the size of the system i.e. 16 x 245W solar PV panels = 3.92kWp
  2. The electricity produced by the solar PV system kWh (kilowatt hours/units) / per year is: 0.8 x kWp x S x ZPV

0.8 = Reducing factor (Solar energy the UK receives = 800W/meter for every 1000W/meter transmitted)
S = Annual solar radiation factor from table 1
ZPV = Over shading factor from table 2

Table 1: Annual Solar Radiation (kWh/m2)

Tilt of Collector Orientation of Collector
South SE/SW E/W NE/NW North
Horizontal 933
30 degrees 1073 1027 913 785 730
45 degrees 1054 997 854 686 640
60 degrees 989 927 776 597 500
Vertical 746 705 582 440 37

Table 2: Over shading factor

Over Shading % of Sky Blocked by Obstacles Over Shading Factor
Heavy >80% 0.5
Significant >60% – 80% 0.65
Modest >20% – 60% 0.8
None or Very Little >20% 1

Example: South Facing Roof, 45 degrees No Shading

3.92kWp x .08 x 1054 x 1 = 3.305kWh/Year