Home Energy Audit Tool

How this audit estimates your EPC band

A real Energy Performance Certificate is produced by an accredited assessor running the government's SAP (Standard Assessment Procedure) software: they measure every room, inspect the boiler's data plate, check insulation where it is visible, and lodge the result on the national EPC register. This tool compresses that process into fifteen questions and a simplified scoring model — "SAP-lite" — that starts every home at 100 points (a perfect new-build) and subtracts points for each weakness using weights that mirror SAP's priorities. The exact weights are documented in the page's code comments: walls and age carry the heaviest deductions because in SAP they dominate heat loss, while lighting and extractor fans are small because in SAP they are small.

The resulting score maps to EPC bands on the standard SAP scale: A at 92+, B at 81+, C at 69+, D at 55+, E at 39+, F at 21+, and G below 21. The average UK home sits in band D (around 60–65 points), which tells you something important: if this audit puts you in D, you are normal, and the measures below will move you up faster than you might expect. A jump from D to C is the single most valuable band move in the UK property market — it is the threshold mortgage lenders use for green products and the minimum standard the government has repeatedly proposed for rentals.

Why the questions are in this order

The quiz starts with the things you cannot change cheaply — property type and age — because they set the ceiling. A pre-1920 detached house with solid walls begins life 33 points behind a post-2012 flat before a single question about behaviour, and no amount of LED bulbs closes that gap; only wall insulation does. That is deliberate: the audit is a fabric-first tool, and fabric questions (walls, loft, floor, glazing, draught-proofing) account for the majority of available deductions. Heating questions come next because the system matters enormously but is downstream of fabric — putting a heat pump in an uninsulated solid-wall house is the classic expensive mistake, and the scoring reflects it by punishing poor fabric more than an old boiler.

The final questions are the quick wins: cylinder jackets, lighting, draught-proofing. They score few points because they save modest energy in SAP terms, but they appear in recommendations precisely because their payback is measured in months, not decades. The audit's recommendation engine works by ranking your worst answers — the largest deductions — and mapping them to measures with Energy Saving Trust typical cost and saving bands. Your top three priorities are therefore the three weaknesses costing you the most, which is also how a professional retrofit assessor thinks.

Reading your result honestly

Three things this audit cannot do. First, it cannot see inside your walls: if you answer "not sure" on wall construction, it assumes the worse case, which is the safe assumption for planning but may underrate your home. A borescope survey (£100–£200) settles it. Second, it does not model your behaviour — SAP assumes standard occupancy and heating patterns, so a frugal household will outperform its band and a wasteful one will underperform it. Third, it is not lodged anywhere: landlords need a real EPC from the register, and grant schemes like ECO4 and the Boiler Upgrade Scheme require one too. Treat this audit as a prioritisation tool, then get the real certificate when money is on the table.

What the bands mean for your wallet

The band is not just a letter — it is a gateway. Band C is the emerging dividing line in UK housing policy: green mortgages typically require A or B, proposed minimum rental standards target C, and several grant schemes prioritise E–G homes because that is where the fuel-poor live. Moving from D to C typically needs 10–20 SAP points, which is achievable with two fabric measures (loft insulation plus cavity fill, for example) in many homes — our EPC improvement estimator models exactly this. Moving from C to B is harder and usually needs a heating system change or solar. Moving from G or F upward is where the biggest bill savings live: an F-rated home can easily spend twice per square metre what a C-rated one does.

Why two identical houses can get different EPCs

One dirty secret of the EPC system: the same house can receive different ratings from different assessors. Studies have repeatedly found band-level variation, because assessors make judgement calls — estimating wall construction where it is hidden, dating the boiler from a worn data plate, deciding whether that loft insulation is 100 mm or 150 mm under poor lighting. The system also rewards what is documented: keep receipts, MCS certificates, and insulation guarantees, and show them to your assessor, because an assessor who can evidence a measure scores it higher than one who must assume. If you are preparing for an assessment that matters (a sale, a remortgage, a grant application), tidy the loft hatch access, dig out the boiler manual, and photograph insulation before it is boarded over — these small acts can be worth half a band.

Your top three priorities: how to act on them

The recommendation panel links each priority to its dedicated calculator on this site — run the numbers for your specific home before getting quotes, because national averages lie about individual houses. The general sequencing rule is: quick wins first (anything with payback under two years: cylinder jacket, LEDs, draught-proofing, controls), then fabric (insulation, glazing — the building must be efficient before new heating), then systems (heat pump, solar — sized for the improved fabric, not the leaky original). Our whole-house retrofit planner assembles this into a phased plan with cumulative costs when you are ready for the full picture.

Common surprises in audit results

First surprise: heating controls often outrank a new boiler in the recommendations. A £200 smart thermostat on an old boiler can save more per pound than a £3,000 boiler upgrade — controls are the most underrated measure in UK housing. Second: draught-proofing appears for almost everyone, because almost every UK home has gaps its occupants have stopped noticing; the chimney alone can account for a startling share. Third: if you scored well everywhere but still landed in C, the remaining points are almost always in the heating system — gas boilers cap out, and only heat pumps or solar push into B territory. That is not a flaw in the audit; it is the physics of the UK housing stock. If your result surprised you, re-check the two answers people get wrong most often — wall construction (many 1930s houses have cavities their owners never knew about) and loft depth (compressed old insulation reads deeper than it performs).