Methodology
Every number a tool on this site produces comes from the tables below. Nothing is hidden in a black box.
Band table
The SAP score bands used across the site, as printed on an EPC for England & Wales.
| Band | SAP score |
|---|---|
| A | 92–100 |
| B | 81–91 |
| C | 69–80 |
| D | 55–68 |
| E | 39–54 |
| F | 21–38 |
| G | 1–20 |
Measure costs and points
Indicative installed costs taken from EPC recommendation reports for England & Wales, with the SAP points each measure typically earns. Costs are scaled by property size: 0.7 for small, 1 for average, 1.45 for large. Points are never scaled.
| Measure | Points | Cost | Per point |
|---|---|---|---|
| Hot water cylinder insulation | +2 | £20–£60 | £20 |
| Loft insulation to 270 mm | +4 | £100–£350 | £56 |
| Low energy lighting throughout | +1 | £25–£90 | £58 |
| Cavity wall insulation | +9 | £600–£1,800 | £133 |
| Heating controls and TRVs | +3 | £350–£900 | £208 |
| Condensing gas boiler | +8 | £2,200–£3,600 | £363 |
| Solar PV, 3 kWp | +11 | £5,000–£8,000 | £591 |
| Internal solid wall insulation | +12 | £4,000–£14,000 | £750 |
| Double glazing throughout | +3 | £3,300–£6,500 | £1,633 |
| Air source heat pump | +6 | £9,000–£14,000 | £1,917 |
Expiry
An EPC is valid for ten years from the date it is lodged. We calculate the expiry date as the lodgement date plus ten years, minus one day, so a certificate lodged on 14/03/2016 is valid up to and including 13/03/2026. We warn from 183 days out.
The 2030 standard
Band E is the current minimum for a private rented home in England & Wales. From 1 October 2030 the minimum is band C, with a £10,000 cost cap per property and qualifying spend counted from October 2025. Source: government announcement, January 2026.
An exemption for low value properties has been announced but the formula is not confirmed. We flag properties valued under £100,000 and deliberately do not reduce the cap, because any figure we produced would be a guess.
The rating estimator
The rating calculator uses an additive model: a base score plus a coefficient for each property feature. The coefficients are placeholders derived from RdSAP feature weightings and will be refitted from the bulk EPC dataset for England & Wales. The stated range is the score plus or minus the model’s mean absolute error of 6 points.
It is an estimate, not an assessment. Only an accredited assessor can produce a certificate.
Upgrade paths
The upgrade cost calculator ranks measures by average cost per SAP point and takes them cheapest first until the target band is reached. If every measure is used and the target is still not met, we say so rather than inventing further work.