Thursday, 7 May 2015

The Impact of Solar PV and Solar Thermal on EPC Ratings

The EPC Rating is a score between 1 and 100


The Energy Performance Certificate (EPC) is a fundamental plank of the government’s strategy to improve the energy performance of the UK’s building stock.

Since 2007 it has been a legal requirement that homes for sale have a report of their energy performance for potential buyers.
From 2018 it will be a requirement that rental properties have a rating higher than E.
In Scotland, all social housing will have to achieve an energy rating of C or D (depending on house type) from 2020
To access the Feed in Tariff for solar PV, it is necessary that the building achieves an EPC D rating
To access the domestic Renewable Heat Incentive, it is necessary to undergo a Green Deal Assessment, which is essentially an EPC with added extras to factor in the way you use energy.

The certificate rates buildings with a score from 1 (least efficient) to 100 (most efficient), with the scores divided into bands A through G as shown.

How it is Worked Out?

An approved calculation called the Standard Assessment Procedure (SAP) is used to calculate the energy used to heat the home, provide hot water to occupants together with electricity for lighting, pumps and fans.  Electricity used by other appliances is not considered.

The actual number of people in the house and the way it is heated is ignored.  A standard occupancy and a fixed indoor temperature is assumed – the idea is to compare the building with other buildings, not compare one set of occupants with another.

The energy used (gas, electricity oil) is then multiplied by fuel cost factors to produce a calculated energy cost for the property.  The energy cost is normalised by the floor area of the property to give a score between 1 and 100, the higher the score, the lower the building costs to run (with a house with a score of 100 nominally costing nothing)

For new homes the current calculation is SAP 2012 and the calculations are updated every time the building regulations change.

For existing homes, assessors use RdSAP 2009 (the Rd stands for Reduced Data).  This is an extra appendix to the SAP 2009 calculation which provides guidance on what assumptions to make when you don’t know or can’t see the exact specification of insulation and equipment.

Solar PV

SAP 2009 gives an unshaded, south-facing solar PV installation around 858kWh/year per kWp installed, irrespective of its the location in the UK.

Its calculation of energy costs assumes that 50% of the energy from a solar PV system is used in the dwelling and 50% exported (and this assumption does not change with the size of the PV installation).

The saving on the energy bills is then:

50% x energy generated x cost of purchased electricity
50% x energy generated x payment for exported electricity

In SAP 2009 the cost of purchased electricity is set to 11.46p/kWh for homes on a standard tariff, and the payment for exported electricity is also set at 11.46p/kWh.

Solar Thermal

A solar thermal installation could benefit the household energy bills in two ways.  First the solar system will generate heat that the boiler or electric immersion heater no longer needs to supply.  Second, a new solar hot water cylinder with better insulation will result in a reduced escape of heat from the stored water.  Although this heat loss contributes towards space heating in winter, but is wasted energy in summer.

SAP 2009 calculates out a solar energy input of 1,316 kWh per year for an unshaded 4 square metre flat plate installation, facing south and heating a 250 litre cylinder in an 85m2 house.  This translates into a fuel saving of 1,586kWh when the replaced heating system is a gas or oil boiler (taking into account their lower efficiency in summer months).

In addition the replacement of an old cylinder (where there is one) with a new would reduce heat losses.  For example, replacing a 50mm jacket insulated 180litre cylinder with a new 210 litre solar cylinder with 105 litre auxiliary heated volume and declared loss of 1.8kWh/day will save 444kWh/year based on a boiler winter efficiency of 90%.

Impact on EPC Rating

The impact on EPC rating of solar PV and solar thermal was calculated as follows.  For every SAP 2009 rating from 1 to 100, the implied Energy Cost Factor (ECF) was calculated by rearranging equations (10), (11) and (12) –  Section 12, page 33.  A house of 85 m2 floor area was considered, being the UK average size.

The energy cost that had resulted in that SAP rating could then be calculated from equation (357).

The saving on the energy cost was calculated by multiplying the energy savings from solar (discussed above) by the fuel prices in table 12.  The reduced energy cost was then converted back to a SAP rating.  The table shows the improvement in energy performance score for solar thermal and PV.

The impact of solar on a home's EPC energy score
NOTE: Figures shown are for a starting EPC score of 40 or higher (beginning of EPC band E), below this the size of the improvement decreases a little.


There are a growing number of drivers that are pushing building owners to improve the energy performance of their buildings.

Until recently, much of the focus has been on insulation measures to achieve these goals, but as more and more of the available cavity walls and lofts have been treated, the remaining insulation options such as external wall, internal wall and under-floor become disruptive and costly.

Solar thermal and solar PV are low hassle – high impact measures that can help increase the energy performance of homes.

Solar PV can give a significant boost to the energy rating of homes, particularly those with a clear roof of adequate size.

Solar thermal can be extremely cost-effective when combined with other heating system works such as boiler or hot water cylinder replacement and is more suitable for smaller roofs and partial shading.

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