How accurate are our figures?
Every figure on this site comes from one published model. On 24 September 2026 we tested that model against the largest sets of measured data available for UK homes. Two parts of it were wrong, and a third, our insulation savings, was far more optimistic than what homes actually saw. We changed all three. This page shows what we tested, what we found and what we changed.
The short version
- How much heat a home uses: built from and checked against government meter data for 39,502 gas heated homes. Within 0.3 per cent of the published medians for 2 to 5 bedroom homes.
- Heat pump efficiency: our average of 2.9 was 4 per cent above the 2.78 measured across 428 real heat pumps. Now matched to the measurements.
- Heat pump installed cost: £1,400 to £4,100 below what installers recorded for the same size of heat pump across 30,590 grant funded installations. Now matched by size.
- Insulation savings: our insulation calculator showed two to four times what homes measurably saved. It now shows the measured savings.
- Not yet tested: whether our figure for one particular home matches that home's own bills and quotes. That needs metered data from real households, and we say so wherever it matters.
How much heat a home uses
The model starts from how much gas a home burns in a year. That comes from the government's National Energy Efficiency Data-Framework (NEED), which links meter readings to property records: 39,502 gas heated homes with 2024 consumption, fitted by property type and floor area. We then checked the result against NEED's own published median gas use by number of bedrooms. It lands within 0.3 per cent for 2 to 5 bedrooms and 3.3 per cent for 1 bedroom.
An honest caveat: this is a calibration, not an independent test, because the checking figures come from the same data. It shows the model reproduces what the typical home of each kind uses. It does not show how close it gets for one particular home, which varies a great deal with how people heat.
Heat pump efficiency
A heat pump's running cost depends on its seasonal efficiency: how many units of heat it delivers for each unit of electricity over a year. We had assumed 2.6 to 3.4 depending on how well the home is insulated. The government funded Electrification of Heat trial measured 428 air source heat pumps in real homes over a full year. Measured across the whole system, including the immersion heater, backup heater and pumps, which is what a household pays for, the median was 2.78, and the middle half of homes sat between 2.55 and 3.05.
| Insulation | We used | We now use | Measured position |
|---|---|---|---|
| Poor | 2.6 | 2.55 | lower quartile, 2.55 |
| Average | 2.9 | 2.78 | median, 2.78 |
| Good | 3.2 | 3.05 | upper quartile, 3.05 |
| Excellent | 3.4 | 3.25 | about the 90th percentile |
For a 3 bed semi with average insulation, that moves the yearly heat pump running cost from £899 to £937 on a standard tariff, and from £614 to £641 on an 18p heat pump tariff. An independent analysis of Ofgem's heat pump data, cited in the same report, found a median of 2.74 on the same measure for heat pumps installed from 2022 onwards.
Heat pump installed cost
Every grant funded heat pump in England and Wales is recorded with the price the installer charged. The government publishes the median by size of heat pump: £13,021 across 30,590 air source heat pumps in 2025/26, including the system, labour and VAT, before the grant. We compared our figure for each home with the median for a heat pump of the size our model gives it.
| Home | Size | We used | Recorded median, nearest size band | We now use | Middle half of installs |
|---|---|---|---|---|---|
| 2 bed flat | 6 kW | £7,700 | £12,164 (6 to 8 kW, 8,054 homes) | £11,800 | £10,000 to £14,500 |
| 2 bed mid-terrace | 5.5 kW | £8,700 | £11,494 (4 to 6 kW, 4,206 homes) | £11,700 | £10,000 to £14,000 |
| 3 bed end-terrace | 8.5 kW | £10,000 | £12,686 (8 to 10 kW, 8,526 homes) | £12,600 | £11,000 to £15,500 |
| 3 bed semi | 9 kW | £10,500 | £12,686 (8 to 10 kW, 8,526 homes) | £12,700 | £11,000 to £15,500 |
| 3 bed detached | 9.5 kW | £11,600 | £12,686 (8 to 10 kW, 8,526 homes) | £13,000 | £11,000 to £15,500 |
| 4 bed detached | 12 kW | £12,500 | £15,365 (12 to 14 kW, 2,824 homes) | £14,700 | £12,500 to £18,000 |
| 5 bed detached | 16 kW | £13,400 | £14,817 (16 to 18 kW, 956 homes) | £16,100 | £14,000 to £19,500 |
We read between the size bands rather than rounding to one, so a 6 kW heat pump sits between the 4 to 6 kW and 6 to 8 kW medians. The 16 to 18 kW band has a lower median than the 14 to 16 kW band, on a smaller sample of 956 homes, so we read past it to keep cost rising with size. The middle half of installs is the scheme's own lower and upper quartile, scaled to each median.
Our figures were too low across the board, by the most for small homes, because much of the cost of an install is fixed and does not shrink with the house. For a 3 bed semi the calculator's figure moves from £10,500 to £12,700, and the cost after the £7,500 grant from £3,000 to £5,200. Homes replacing oil or LPG now get the £9,000 grant available to them until March 2027.
Insulation savings
NEED also measures what homes saved after insulation, by comparing their gas use before and after against similar homes that had nothing done. Those measured savings are much lower than the modelled figures most calculators use, including ours, because many homes were under-heated before and take part of the gain as warmth rather than a lower bill.
| Measure, 3 bed semi on gas | We showed | Measured median saving |
|---|---|---|
| Cavity wall insulation | £215 a year | £105 a year (12.0%, 2,191 homes) |
| Solid wall insulation | £340 a year | £151 a year (17.3%, 2,411 homes) |
| Loft insulation (top-ups and bare lofts) | £120 a year | £28 a year (3.2%, 1,486 homes) |
The insulation calculator now shows what homes like yours typically saved, and says plainly that a home heated the same way before and after can save more. Its installation costs now follow the Energy Saving Trust's 2026 figures, which were well above ours.
What we have not tested yet
- Your home against your bills. Every figure here describes the typical home of its kind. How close it gets for a particular house depends on how that household heats it, and testing that needs metered data from real homes, shared with consent.
- The EPC estimator. It is a simple points model, not an assessment, and we have not compared it with lodged certificates.
- Solar generation. We use the European Commission's PVGIS model for generation. NEED measured a median 12.2 per cent fall in electricity bought after solar was fitted, which we have not yet reconciled with our figures.
How we keep the site in line with the model
The model is one readable file, heat-model.js, and the methodology explains each step. Before any change goes live, automated checks read the heat demand stated on 18 guides and 154 cells in 8 property tables and compare each one with the model. A page that disagrees fails the same check as a broken link.
Sources
- Department for Energy Security and Net Zero, Boiler Upgrade Scheme statistics, August 2026, Tables A1.3A and Q1.1A, published 24 September 2026.
- Energy Systems Catapult for the Department for Energy Security and Net Zero, Electrification of Heat Demonstration Project: Heat Pump Performance Data Analysis Report, December 2024, Tables 1.2 and 8.1.
- Department for Energy Security and Net Zero, NEED impact of measures data tables 2026, Table 1, published 11 June 2026, and the NEED 2026 consumption data.
- GOV.UK, Apply for the Boiler Upgrade Scheme, and the £9,000 grant for oil and LPG homes, 27 August 2026.
- Energy Saving Trust, cavity wall, solid wall and loft insulation advice pages, updated 10 February, 18 June and 8 May 2026.
Tested 24 September 2026. When a source updates, we re-run the comparison and this page changes with it.