EPC ratings explained

As an Energy Assessor, I am often asked by clients why their homes have achieved certain scores on their EPC (Energy Performance Certificate). We all want to achieve the highest possible score and sometimes it can be disappointing to be awarded a lower than expected rating. An EPC rating ranges from A to G, with A being the best possible, G the worst. The letter rating is awarded based on the numerical score from 1 upwards, as can be seen in the EPC ranking below as you would find it on an official certificate. Any existing EPCs can be viewed for free on the government website https://find-energy-certificate.service.gov.uk/

How is the numerical score calculated?

The numerical score that influences the final EPC rating is calculated during the full energy assessment which uses a program called SAP (Standard Assessment Procedure). The SAP uses complex calculations that work in the background to assess how much energy a dwelling uses (kwh/yr), how much it will cost to heat and how much CO2 it is likely to emit annually.

The assessor enters in the dimensions of the house, the heating system, the u-values (how quickly heat passes through a solid) of the physical elements, thermal bridging as applicable (heat loss along junctions such as where a floor meets a wall) any renewables such as solar panels, air pressure test and ventilation system. This eventually gives the final score, which is essentially the cost of heating the dwelling annually divided by the floor area. Therefore you can surmise that using more energy efficient heating systems and having a well insulated home will reduce this number and the final EPC score.

I expected my score to be better - why isn't it?

When I am asked this question, I find it easier to instead explain what would be needed to achieve a good EPC score. Often on existing homes it can be difficult to get them to a new build standard and so the high B+ scores are difficult to achieve without some very costly improvements. Against todays Building Regulations, a home usually needs to be hitting all of the following for a B and higher:

• All U values under 0.18 for walls

• All u values under 0.16 for roofs

• All u values under 0.18 for floors

• U values for all doors and glazing under 1.4

• Thermal bridging details used and measured for all junctions (this is one that has a significant effect on things)

• An air test of 5 or lower

• Heat pump heating

• Renewables such as solar panels

With any project, it is always advisable to seek the advice of an Energy Assessor as early as possible in the process so you can discuss any EPC needs and make sure you are hitting them.