Calculation of the EPC Energy Rating
Chester Energy Assessors
The EPC band ratings
The EPC calculates both an energy rating and an environmental rating for your property, as well as potential ratings if you were to carry out the more cost-efficient improvements that are recommended - see an example epc on a 1930's semi in Chester (appears in a separate window). The energy rating is a measure of the energy costs per square metre of floor space, while the environmental rating is a measure of carbon dioxide emissions per square metre. Because they are related to the area of your home, a house that is equally energy efficient and built to the same standards as another, but is twice the size, will have a similar band rating but will have twice the actual fuel costs.
The energy ratings and environmental ratings tend to move together, and for natural gas-heated homes they are often quite similar. They range from a very poor G1 rating, such as you might get with a small detached Victorian cottage with no loft insulation, no hot water tank insulation and no central heating, to an A (91-100), which is possible for a very modern middle-floor, mid-terrace flat (i.e. adjacent flats above, below and on either side) with extensive renewable energy provision such as solar water heating and photovoltaic cells. Higher than 100 is theoretically possible if you produce more energy than you consume and export excess electricity back to the National Grid.
Typical EPC energy ratings in Chester and surrounding areas
In the Chester area there is very low uptake of renewable energy and the best rating that Chester Energy Assessors has seen in Chester is for a flat in Hopkinson Court near the racecourse. This was a C78 energy rating, and with appropriate improvements but not renewable energy installation, it could be brought up to a B83. Outside Chester, the best energy rating seen has been a B84 for a 2007-built top-floor flat in Lamberton Drive in Brymbo near Wrexham. The average energy rating we've seen overall has been an E band (E53), but nationally it is slightly higher at a low D. This presumably reflects the older than average housing stock in Chester, but its worth pointing out that we've seen many pre-war semi-detached houses and Victorian terraces that have achieved a high D rating.
Calculating the EPC energy rating
Standard software is used to calculate the energy and environmental ratings using the data inputted by the energy assessor (DEA). The software uses a calculation methodology laid down by law and developed by the BRE Group called RDSAP (Reduced Data Standard Assessment Procedure), which inevitably uses some assumptions in order to compare different houses on the same basis. It first looks at the volume of the house and how much energy would be required to heat the different areas to standard temperatures (e.g. 18 °C for the hall, 21 °C for the living areas) for a certain number of hours per day.
Then it looks at how the energy is generated and how efficient the heating systems are. For instance, a modern condensing gas boiler is 91% efficient, whereas an early 1990's boiler is typically 65%, meaning that a third of the gas is wasted before we start. Finally, the software calculates how quickly the heat is lost through the external walls and roof of the house, which is where the importance of insulation comes in. Other lesser issues which go into the calculation are the level of hot water tank insulation, the number of low energy light bulbs, and the temperature controls on the heating system. Finally, it calculates the energy costs in running the house, divides it by the total floor area then converts the figure to an energy rating.
Energy costs calculated on the EPC and effect of different fuels
On the first page of the EPC under the coloured bar charts are various other data, including a breakdown of annual energy costs separately for heating, lighting and hot water. These are calculated figures, assuming the property is used and heated in a standard fashion as described above. Its interesting to compare them with the real fuel bills for the year. In practice, occupiers habits will widely differ and the calculated figure can be very different from reality. In the example EPC referred to at the top of the page, the real fuel bills were actually 20% less than calculated. Another factor is that the prices per kWh of fuel assumed by the software could be quite different to what the customer is actually paying.
The cheapest and most common fuel is natural gas. Electricity (even Economy 7), oil, and LPG are all considerably more expensive. Using one of these alternative fuels for heating greatly increases the energy costs and can put the energy rating of the property 1 or 2 bands below an equivalent one heated by natural gas.
