An EPC models a standard household in a standard pattern of use, so it is a fair way to compare two buildings and a poor way to predict one family's bills. Two identical houses with the same band can differ by hundreds of pounds a year purely on how the people inside live.
What the model assumes vs what actually happens
| The assessment assumes | Real households |
|---|---|
| A standard heating pattern and target temperature | Heat one room, or the whole house, at very different temperatures |
| Standard occupancy for the floor area | Two people in a four-bed, or six in a two-bed |
| Standard hot-water demand | Varies enormously between households |
| A fixed set of fuel prices at assessment date | Prices move; the certificate does not update |
This is why an EPC is best used comparatively. "This house is a D and that one is a B" is a meaningful statement about the buildings. "This EPC says £1,400 a year so that is my bill" is not.
It is also why the certificate can be out of date without being wrong: it is valid for ten years, so a new boiler or new insulation fitted since will not be reflected until a fresh assessment is done.
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Last reviewed: 2026-09-20. We check these against the linked official sources and re-date the page when they change.