How it is Worked Out?

The energy consumed to heat the home, give hot water to the occupants, as well as electricity for lighting, pumps, and fans, is calculated using the Standard Assessment Procedure (SAP). Other appliances’ electricity is not taken into account.
The amount of people in the residence and how it is heated aren’t taken into account. The goal is to compare the building to other buildings, not one set of occupants to another, hence a standard occupancy and fixed inside temperature are assumed.
The energy consumed (gas, electricity, or oil) is then multiplied by fuel cost variables to get the property’s energy cost. The energy cost is normalised by the property’s floor area to get a score between 1 and 100; the higher the score, the lower the building’s operating costs (with a house with a score of 100 nominally costing nothing)
The current calculation for new dwellings is SAP 2012, and the calculations are revised if the construction regulations change.
Assessors utilise RdSAP 2009 for existing homes (the Rd stands for Reduced Data). This is an addendum to the SAP 2009 computation that explains what assumptions to apply when you don’t know or can’t see the actual insulation and equipment specifications.

Solar Thermal

In two ways, a solar thermal installation could reduce household energy bills. First, the solar system will create heat that will eliminate the need for the boiler or electric immersion heater. Second, a new solar hot water cylinder with greater insulation will prevent heat from the stored water from escaping. Although this heat loss helps with space heating in the winter, it is a waste of energy in the summer.

For an unshaded 4 square metre flat plate installation facing south and heating a 250 litre cylinder in an 85m2 residence, SAP 2009 calculates a solar energy input of 1,316 kWh per year. When the new heating system is a gas or oil boiler, this equates to a fuel savings of 1,586kWh (taking into account their lower efficiency in summer months).

Furthermore, replacing an old cylinder (if one exists) with a new one would reduce heat losses. Based on a boiler winter efficiency of 90%, replacing a 50mm jacket insulated 180litre cylinder with a new 210 litre solar cylinder with 105 litre auxiliary heated capacity and stated loss of 1.8kWh/day will save 444kWh/year.

Solar PV

SAP 2009 estimates that an unshaded, south-facing solar PV plant will generate around 858kWh/year per kWp installed, regardless of location in the UK.

It bases its energy cost calculations on the assumption that 50% of the energy generated by a solar PV system is used in the home and 50% is exported (and this assumption does not change with the size of the PV installation).

The energy bill savings are as follows:

50% x generated energy x cost of imported electricity
+
50% x generated energy x payment for exported electricity

In SAP 2009, the cost of purchased electricity for residences on a regular tariff is set at 11.46p/kWh, and the cost of exported power is also set at 11.46p/kWh.

Impact on EPC Rating

The influence of solar PV and solar thermal on the EPC rating was computed as follows. The inferred Energy Cost Factor (ECF) was computed by rearranging equations (10), (11) and (12) – Section 12, page 33 – for each SAP 2009 rating from 1 to 100. The average size of a house in the United Kingdom was 85 m2.
Equation may then be used to compute the energy cost that resulted in the SAP rating (357).

Multiplying the energy savings from solar (described above) by the fuel prices in table 12 yielded the energy cost savings. After that, the lower energy cost was converted back to a SAP rating. The table depicts the improvement in solar thermal and PV energy performance scores.