Which? selected 12 of its members to take part in mystery shopping research and have an energy assessment carried out at their homes in February and March 20241. The consumer organisation paid between £47 and £85 for the assessments1. Of the 11 participants who completed the process, eight said their Energy Performance Certificate (EPC) did not appear to be accurate, and only one was "very satisfied" with it1. Three said they were likely to recommend getting an EPC based on the experience1.
An EPC gives a home a rating between A (very efficient) and G (inefficient)1. Certificates were introduced in 2007 as part of the Home Information Pack, and around 65% of UK homes have had at least one1. They are valid for ten years, so an existing certificate may have expired1. A new one costs around £60 to £120 depending on the size and location of the property1. Only qualified Domestic Energy Assessors can produce EPCs for homes, and all of them must join an approved accreditation scheme through which each certificate is lodged1. The accreditation schemes are required to audit at least 2% of all EPCs lodged1.
The research follows earlier work on accuracy. In 2019, researchers at Leeds Beckett University found issues with 27% of EPCs that suggest they are incorrect1. Which? sets out several reasons why a certificate may not match a household's experience. An assessment is non-invasive, so no holes are drilled and no cavities are opened up for inspection, meaning assumed ratings are used where an assessor cannot inspect an area or feature1. The software records more than 140 pieces of data, but only a simple summary appears on the certificate1. The rating is calculated on how much the home will cost to run, and gas is the cheapest heating fuel available, so gas boilers tend to receive the highest ratings1. As a result, current EPC recommendations never include switching to a heat pump1.
The certificate only considers heating, hot water and fixed lighting, ignoring energy use from kitchen appliances, televisions, computers and other energy-using devices1. Predicted costs use "standard occupancy", which assumes household size according to the number of bedrooms and a fixed heating pattern and thermostat temperature1. Costs quoted are based on typical energy costs at the time the EPC is issued1.
"Eight out of 11 told us their EPC did not appear to be accurate"
Why it matters for households
An EPC is the document that records a home's energy performance, and it is used in decisions about buying, renting and improving a property. If the rating or the underlying assumptions are wrong, the picture a household has of its own home may be wrong too. The research indicates that this happened in most of the cases Which? followed, though the sample was small.
The limits of the certificate matter for insulation and glazing decisions. Features that are difficult to inspect, such as floor insulation, internal wall insulation, flat roof insulation and loft insulation where there is no loft access, may be recorded as assumed rather than confirmed1. Which? says documentation or photographs showing insulation or other building works can be taken into account by the assessor1. The certificate is not a condition survey, so defects such as a broken window or an incorrectly installed boiler are not mentioned or factored into the calculations1.
For households thinking about installers and assessors, the certificate's recommendations are shaped by running costs rather than carbon alone. A heat pump reduces carbon emissions but is unlikely to improve an EPC energy rating because electricity is more expensive than gas, which is why current recommendations never include switching to one1. Landlords cannot legally rent out properties with the worst ratings, F or G1.
What happens next
Which? says that if there is a mistake, the accreditation scheme should correct it and issue a new EPC at no extra cost1. No further dates or next steps have been reported.