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RED Electrical - Worked Example

Example - a 4kW well-sited solar PV system

In the following example, a 4kW solar PV system produces enough electricity per year to make its owner £700 in Clean Energy Cashback. The surplus electrictity sold back to the supplier is bringing in an extra £52, and the money saved by not buying electricity from the supplier is £233.

Total financial benefit: £985 over the year!

Prove it!

In order to prevent misleading information, the Government has developed a Standard Assessment Procedure (SAP) which gives us a method of calculating the annual electricity generation. The full SAP document can be downloaded from here. The formula provided by SAP is:

Electricity produced = 0.8 x kWp x S x Z_PV

kWp is the peak power of the solar PV system.
S is the annual solar radiation; refer to table H2 at the bottom of the page (source: SAP2009).
Z_PV is the overshading factor; refer to table H4 at the bottom of the page (source: SAP2009).

For a south-facing system with 30° roof pitch that is not overshadowed by trees etc, the formula is:

0.8 x 4.0 x 1042 x 1 = 3334.4 kWh per year

For a retro-fit system (i.e. added after the house is built), the price paid by the electricity supplier for this size of system is 43.3p per kWh. The export rate (i.e. the unused electricity you sell back to the supplier) is 3.0p per kWh, and assuming the usual retail cost of electricity is 13.0p per kWh, the financial benefits are as follows:

Clean energy cashback:	3334 x 21p	700.22
Export (assuming 50% electricity sold):	50% x 3334 x 3.1p	51.68
Savings (assuming 50% electricity used):	50% x 3334 x 14.0p	233.41
	Total financial benefit for year	£ 985.32

Table H2 - annual solar radiation, kWh/m²

Tilt of collector	South facing	SE/SW facing	E/W facing	NE/NW facing	North facing
Horizontal	933	933	933	933	933
30°	1042	997	886	762	709
45°	1023	968	829	666	621
60°	960	900	753	580	485
Vertical	724	684	565	427	360

Table H4 - overshading factor

Overshading	Heavy	Significant	Modest	None or very little
% of sky blocked by obstacles	>80%	>60% - 80%	20% - 60%	<20%
Overshading factor	0.5	0.65	0.8	1.0

Example - a 4kW well-sited solar PV system

Total financial benefit: £985 over the year!

Prove it!

Electricity produced = 0.8 x kWp x S x ZPV

0.8 x 4.0 x 1042 x 1 = 3334.4 kWh per year

Electricity produced = 0.8 x kWp x S x Z_PV