How the plan works
The plan starts from how much fuel a home like yours uses in a year, from the government's National Energy Efficiency Data-Framework: meter readings for 39,502 gas heated homes, by property type and size. It then lists the insulation your home may need, cheapest payback first, and takes each saving from what is left after the one before, so two 10 per cent savings make 19 per cent, not 20. Last comes a heat pump, sized and costed for your home as it will be after that insulation.
Where the savings come from
Every insulation saving is the median fall in gas use the government measured in real homes after that measure was fitted: 12.0 per cent for cavity wall insulation across 2,191 homes, 17.3 per cent for solid wall insulation across 2,411, and 3.2 per cent for loft insulation across 1,486, most of them top-ups. These come from NEED's Impact of Measures tables, published in June 2026. They are lower than most calculators show, and that is deliberate: they are what homes actually saw.
Where the costs come from
Insulation costs are the Energy Saving Trust's 2026 figures for a typical home: £750 for a bare loft, £600 for a top-up, about £2,700 for cavity walls and about £12,000 for internal solid wall insulation. The heat pump is the median cost installers recorded under the Boiler Upgrade Scheme in 2025/26 for a heat pump of your home's size, less the £7,500 grant, or £9,000 if you are replacing oil or LPG. Running costs use the Ofgem price cap for October to December 2026, and heat pump efficiency is the median measured across 428 homes in the government's Electrification of Heat trial. See how accurate these figures are.
What the plan leaves out
Solar panels depend on your roof, so they have their own calculator. Draught-proofing, windows and heating controls are not in the plan yet because we do not have measured savings for them. Your EPC rating is estimated by the EPC planner, and grants that could cover the insulation are in the grant checker.
For lenders and partners
The plan is one short script over a published table, with no network calls and no personal data. An embeddable version that sits inside another site is at /embed/retrofit-plan/; get in touch to use it.
Frequently asked questions
Why are the savings lower than other calculators show?
Because they are measured. The government compared the gas use of real homes before and after each measure was fitted, against similar homes that had nothing done. Measured savings come in lower than modelled ones because many homes were under-heated before and take part of the gain as warmth. A home heated the same way before and after can save more than the figure shown.
Why does insulation come before the heat pump?
Insulating first lowers the heat a home needs, which lowers what a heat pump costs to run and can mean a smaller unit. The plan works out the heat pump for your home as it will be after the insulation above it.
Are the costs quotes?
No. Insulation costs are the Energy Saving Trust's 2026 figures for a typical home, and the heat pump cost is the median installers recorded under the Boiler Upgrade Scheme for a heat pump of your home's size. A survey turns these into a price for your house, and grants can cover some or all of the insulation.
What is not in the plan?
Solar panels, which depend on your roof, so they have their own calculator. Draught-proofing, windows and heating controls, which we do not yet have measured savings for. And your EPC rating, which the EPC planner estimates separately.