DESNZ has published a report on the much-maligned Energy Performance Certificates (EPCs), that often differ from actual household energy use and have been widely criticised for providing a misleading picture of efficiency.

The DESNZ report summarises evidence from the EPC Accuracy project on why EPC-modelled energy use differs from metered use and what this means for future EPC reform and the development of the Home Energy Model (HEM). Despite the known failings, EPCs have been embedded into Government efficiency regulations, and are likely to continue to be used, even in a modified form.

Using smart meter data, temperature monitoring, and survey evidence from more than 1,100 homes, the report identified the size of the gap. The study found that EPCs overestimate energy use in gas-heated homes by 16 per cent on average. Electrically heated homes use around 31 per cent less energy than predicted, with the gap widening in winter. The main causes are outdated EPC records, modelling assumptions on heat loss, ventilation, temperatures and solar gains, and limitations in how electrically heated homes are represented.

The report recommends making EPCs more relevant by linking them to verified upgrade data so ratings update when major efficiency measures or heating-system changes are installed; revising the core modelling assumptions, especially for heat loss, ventilation, solar gains, electricity demand, and zoned heating behaviour, using empirical validation and improving data quality and assessor accuracy by giving assessors access to better source data and strengthening quality assurance for critical property inputs.

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