The tariff your assessor might have recorded wrong

When a landlord commissions an EPC, they expect the result to reflect their property. What they may not realise is that one data point — the electricity tariff — can move the rating by one or more bands, and assessors sometimes get it wrong.

For properties with storage heaters on Economy 7, an incorrect tariff recording does not just shave a few points off the score. It can mean the difference between Band D and Band C. And because the error is in the data rather than the fabric, correcting it requires no physical work at all.

Why the tariff matters so much

The SAP methodology that underlies every EPC calculates a score based largely on estimated running costs. Electricity costs significantly more per unit than mains gas, which is why electrically-heated properties tend to score poorly — the same amount of heat costs more to deliver, and the rating reflects that.

Economy 7 exists to change that equation. It provides electricity at a reduced rate for approximately seven hours overnight — the exact window varies by supplier and region, but typically falls in the early morning hours. Storage heaters are designed to charge during those hours and release heat through the day. When assessed correctly, a property with Economy 7 storage heaters scores substantially better than the same property on standard-rate electricity.

The difference is not marginal. Depending on the property type, size, and how much of the heating load runs on off-peak electricity, the tariff recognition alone can move a rating by one band or more.

How the error happens

RdSAP — the methodology used for existing properties — requires the assessor to record the heating system type and the tariff associated with it. For storage heaters, the correct recording is Economy 7 (or Economy 10 where applicable). If the assessor records standard rate instead, the software calculates the running costs at the higher tariff and produces a lower score.

This can happen for several reasons. Storage heaters are not always obviously labelled. Older units may not be immediately identifiable as off-peak systems. Landlords are not always present at the assessment and cannot confirm the tariff in use. And if the assessor does not see a visible indicator — a meter with two registers, a visible Economy 7 sticker, documentation from the supplier — they may default to standard rate.

The result is an EPC that understates the property’s rating. Not because the property is poorly insulated or inefficiently heated, but because a single input field was recorded incorrectly.

How to check

The first step is to read the existing EPC carefully. The lodged certificate lists the inputs the assessor recorded. Look for the heating system description — it should identify the storage heaters and the tariff. If it shows standard electricity rather than Economy 7, and the property does have Economy 7 storage heaters, the rating is likely understated.

Confirming the tariff is straightforward. The electricity meter will have two registers if Economy 7 is in place — one for peak consumption, one for off-peak. The supplier can also confirm in writing what tariff is active on the account. Both forms of evidence support a reassessment.

The fix is a new EPC. The assessor returns, records the correct tariff, and the software recalculates. There is no physical work involved. The only cost is the reassessment fee.

What Economy 7 does not fix

Tariff correction is not a route to Band C for every property with storage heaters. The improvement applies specifically to the running cost calculation — it does not change the fabric inputs, the heating system efficiency, or the hot water provision. A property with poor wall insulation and no loft insulation will score better with the correct tariff, but may still fall short of Band C.

The correct approach is to check the current EPC inputs first. If the tariff is wrong, correcting it is the obvious first step — it costs the least and may close the gap entirely. If the tariff is correctly recorded and the property is still Band D, the route to Band C lies in the fabric and heating inputs, not the tariff.

Economy 7 storage heaters also have a specific constraint worth noting: the off-peak charging window must align with actual usage patterns for the tariff benefit to translate into lower running costs in practice. The EPC recognises the tariff; it does not guarantee the tenant is using the system correctly. That is a separate issue, but one worth addressing alongside the rating.

The broader point

EPC ratings depend entirely on what the assessor records. Most inputs are straightforward — wall construction, loft insulation depth, window type. But some inputs, including fuel tariff, depend on information that is not always visible at the time of survey. That creates the possibility of errors that are entirely correctable without any work being done to the property.

For landlords with storage-heated properties that are sitting at Band D or low Band E, checking the tariff recording is the first thing to do before planning any retrofit expenditure. If the rating is wrong, spending money on insulation to close a gap that does not exist is exactly the kind of mistake the existing EPC was supposed to prevent.