An EPC vs Your Actual Energy Bills

An EPC estimate and a real bill measure different things. The EPC models a standard household, heating to standard temperatures for standard hours, at fuel prices current at assessment. A real bill records a real household, heating as it actually does, on its actual tariff, in actual weather. A gap between them is therefore expected, not evidence that either document is wrong, and the size of a normal gap can be large.

A household that works from home, heats every room all day, and keeps high temperatures will pay more than the certificate estimates. A household out all day, heating only evenings, and keeping lower temperatures will pay less. Neither household has disproved the certificate; both have simply lived differently from the standard household the model assumed.

Four factors drive most gaps. Occupancy and behaviour come first: heating hours, temperatures, room use and hot water use vary enormously between households. Tariffs come second: a household on an expensive tariff pays more for the same kilowatt-hours than a neighbour on a cheap tariff, and standing charges add a fixed element the certificate's energy figure does not describe in the same way.

Weather comes third: a cold winter raises real use above a standard-year model, while a mild winter lowers it. Building changes come fourth: work done since assessment, such as new windows or a new boiler, changes real use without changing an old certificate, while deterioration, such as failed window seals or a boiler losing efficiency, pushes real use above the modelled figure. Separating those four factors is the first step to understanding any gap.

Compare energy, not just cost. Convert bills to kilowatt-hours for gas and electricity separately, over a full year, and compare those totals with the certificate's modelled energy figure. If kilowatt-hours are close but cost differs greatly, the gap is tariff, not building. If kilowatt-hours differ greatly, occupancy, weather or building change is more likely at play.

Check readings and estimates. A year of estimated bills followed by a catch-up reading can create a spurious spike that looks like a building problem. Check whether each bill was actual or estimated, whether any fuel was bought in bulk outside metered supply, and whether occupancy changed during the year, such as a household member moving in or out.

In the UK, EPC estimated costs use standard fuel prices and standard heating patterns, and they are presented alongside the rating for comparison, not as a bill forecast. Energy suppliers' bills, by contrast, reflect the household's tariff, region, payment method and any price cap or fixed deal in force. A UK reader should therefore expect cost gaps whenever tariffs differ from assessment assumptions, which is most of the time.

In EU member states, certificate cost presentations vary: some certificates show only energy and carbon, others show estimated costs in national formats with national price assumptions. A reader comparing a certificate cost from one country with a bill from another, or even with a bill in the same country after a tariff change, should treat the certificate figure as a dated snapshot, not a live price.

One mistake is blaming the building for a tariff gap. A household that switches to a cheaper tariff and sees bills fall has not improved the building, and a household whose tariff rises and sees bills rise has not worsened it. Energy improvements should be judged on kilowatt-hours over comparable weather and occupancy, with cost tracked separately as its own story.

Another mistake is blaming the certificate for a behaviour gap. A certificate cannot know that a household heats a conservatory all winter, runs a hot tub, or keeps a home warm for a home office all day. Those uses are real and legitimate, but they belong to the household's energy story, not to the building's modelled story, and they should be accounted for before the building is judged.

Keep a year of bills in kilowatt-hours, with actual versus estimated marked, tariff noted, and occupancy changes recorded, alongside the certificate. That pair of records lets any professional separate building, behaviour, weather and tariff effects, rather than guessing which one explains a gap.

Ask an assessor or energy adviser to help interpret a large gap: which factor most likely explains it, and what check would confirm that. If kilowatt-hours are far above modelled figures after allowing for weather and occupancy, a building check, such as heating controls, boiler efficiency or fabric deterioration, may be warranted.