How to read an EPC: what the rating actually tells you

What the letter actually measures

The A to G rating is an energy efficiency score based on the modelled cost of heating the home, heating its water and lighting it. It is not a measurement of your actual bills. Two identical houses can have very different bills depending on how warm the occupants keep them and how often they are home, but the certificate assumes standard occupancy so that one property can be compared with another.

The score runs from 1 to 100 and higher is better. The bands map onto the score like this.

The two scores people confuse

Every certificate carries a current rating and a potential rating. The potential rating is what the assessor believes the property could reach if the listed improvements were installed. A house at D with a potential of B is a very different prospect from a house at D with a potential of D, because the second one has already had the cheap wins done.

There is also a separate environmental impact rating, which is about carbon emissions rather than cost. A home on mains gas can score well on cost and less well on carbon.

The fabric section is the useful bit

Underneath the headline rating the certificate describes each element of the building and grades it from very poor to very good. Read these lines before anything else.

• Walls: cavity with insulation, cavity without, or solid. Solid walls are the single biggest reason a period home scores badly and the most expensive thing to fix.
• Roof: the depth of loft insulation in millimetres, or a note that the roof room is insulated. Anything under 200mm is worth topping up.
• Windows: single, partial double or full double glazing, and whether the double glazing is modern.
• Main heating: boiler type and fuel. Electric storage heaters, oil and LPG all push running costs up.
• Heating controls: whether there is a programmer, room thermostat and thermostatic radiator valves.
• Floor: often recorded as no insulation assumed, which simply means the assessor could not see under it.

What the recommendations are worth

The recommendations list is generated from a standard set of measures, with an indicative cost range and an estimated annual saving. Treat the costs as a rough guide. They are national averages produced by the model, not quotes, and they can be well out of date for a specific property or a specific year.

What the list is genuinely good for is direction. It tells you which measures the model thinks are still missing, in rough order of value. Loft insulation and cavity wall insulation near the top of the list usually means real, affordable improvement is available. Solar panels and internal wall insulation at the top usually means the easy work has already been done.

How long a certificate lasts

An EPC is valid for ten years. It must be commissioned before a property is marketed for sale or rent, and the rating has to appear in the advert. An old certificate is still legally valid until it expires, but it describes the house as it was on the day of the visit. If a new boiler or new windows have gone in since, the current rating understates the home.

Checking the certificate for any address

Certificates for England and Wales are published as open data, so you can look up the record for a home you do not own, including one you are thinking of buying. That is worth doing before a viewing rather than after, because it tells you what questions to ask while you are standing in the building.