Our Methodology

Figures last reviewed 25 September 2026. Gas and electricity prices are the Ofgem cap for 1 October to 31 December 2026 and change quarterly; the next cap is announced in late November. Heating oil is the UK average on 25 September 2026, and oil prices move.

Every number on Retrofit Planner comes from a published, verifiable source. Here's exactly how our calculators work, what data they use, and where we make assumptions.

Energy prices

Gas and electricity come from Ofgem's quarterly price cap: our current rates are from the cap for 1 October to 31 December 2026, announced on 26 August 2026. Heating oil and LPG are not capped, and what a heat pump tariff costs depends on when the heating runs, so those three are set as shown:

• Electricity: 26.32p per kWh, standing charge 54.83p per day
• Gas: 7.97p per kWh, standing charge 29.68p per day
• Heating oil: 11.3p per kWh, the BoilerJuice UK average of 116.20p a litre including VAT on 25 September 2026, converted at 10.294 kWh a litre (DESNZ greenhouse gas conversion factors 2026, gross calorific value). Oil prices move, so an oil heated home's own bill can be higher or lower than the figures we show.
• LPG: 9.5p per kWh, our assumption for a typical bulk contract
• Heat pump tariff: 18.7p per kWh, Cosy Octopus at its rates from 1 October 2026, read from Octopus's public price feed on 5 October 2026 and averaged across its 14 regions (12.33p in the cheap hours, 25.13p in the day and 37.70p from 4pm to 7pm), with 60% of heating in the cheap hours, 30% in the day and 10% in the 4pm to 7pm peak, exactly as costed in our tariff guide, which gives £666 a year for an average 3 bed semi. A home that kept all its heating out of the peak would pay about 17.5p per kWh.

We update our calculators when Ofgem announces each new price cap (usually late February, May, August and November), so the figures cover the coming quarter. We do not account for future price changes in our calculations because these are unpredictable. All estimates reflect current prices only.

Installation costs

Heat pumps: the median cost installers reported for air source heat pumps paid under the Boiler Upgrade Scheme in 2025/26, by size (DESNZ Boiler Upgrade Scheme statistics, August 2026 release, Table A1.3A, 30,590 installations). Installers report it on the MCS certificate, and it covers the system itself, labour and VAT, before the grant is taken off. We read it at the size our model gives the home, interpolating between size bands. Where we show a range it is the scheme's own lower and upper quartile for April to June 2026 (Table Q1.1A), scaled to that median. Some homes also need larger radiators, which can add to the cost.

Insulation: the Energy Saving Trust's 2026 figures for a typical home: £750 to lay 270mm of loft insulation in a bare loft and £600 to top one up, about £2,200 for cavity wall insulation, and about £12,000 internal or £15,000 external for solid wall insulation on a typical 3 bed semi. Larger homes cost more.

Boilers: about £3,500 for a new condensing gas boiler in a 3 bed home, the DESNZ average including VAT (Boiler Upgrade Scheme 2026 to 2030 summary business case, footnote 8). The boiler comparison scales it by bedrooms, which is our assumption: £2,950 for 1 bedroom, £3,200 for 2, £3,500 for 3, £3,800 for 4 and £4,050 for 5 or more. The EPC planner uses £3,500 for every home.

Heat pump calculations

• Heat demand: from the DESNZ National Energy Efficiency Data-Framework (NEED) 2026, covering 2024 consumption in England and Wales. We took median gas consumption for every property type and floor area band from the 39,502 gas heated records in the anonymised sample, fitted it against floor area for each type, then scaled it by bedroom count so the population weighted result reproduces NEED's own published medians by bedrooms (5,215 kWh for 1 bed up to 19,339 kWh for 5 or more). It matches to within 0.3% for 2 to 5 bedrooms. Every calculator that compares heating costs, and the embeddable widget, loads this one table from /js/heat-model.js. Guide pages state figures derived from it, and checks in the repository compare those figures against the table so they cannot drift unnoticed.
• Insulation adjustment: measured from the same records by comparing EPC bands within each property type and floor area cell, so that size is not counted twice. Across 17 such cells the medians relative to band D are A or B 0.62, C 0.85, D 1.00, E 1.11.
• Efficiency: the whole system seasonal performance measured in 428 air source heat pumps by the government's Electrification of Heat trial (final report, December 2024): median 2.78, with the middle half of homes between 2.55 and 3.05. We use 2.55 for a poorly insulated home, 2.78 for average, 3.05 for well insulated and 3.25 for a recently retrofitted one, our estimate of about the top tenth, since the report gives only the median and quartiles. Linking efficiency to insulation is our assumption: the trial found no significant difference by house type or age, but better insulated homes can run at the lower flow temperatures that raise efficiency. Each source and the date we last checked it is on our data page.
• Size: the home's annual heat demand over 1,100 full load hours, which gives the sizes installers typically fit: about 9 kW for an average 3 bed semi. The right size for a particular home comes from the installer's room by room heat loss calculation.
• Grant: the Boiler Upgrade Scheme pays £7,500 towards an air source heat pump, or £9,000 until March 2027 for homes heated by oil or LPG with no mains gas connection.
• Running costs: heat demand divided by efficiency, multiplied by the electricity rate, compared with heat demand divided by the existing boiler's seasonal efficiency (90% for gas, 85% for oil and LPG, 100% for electric heating) multiplied by that fuel's rate. Both are energy only. Gas also carries a £108 a year standing charge, which a household saves if it caps the gas supply after switching; the calculators show that separately.
• Your own bill: if you enter your bill, we take off the gas standing charge, work out the heat your home actually uses from the rest, and cost the heat pump on that same demand, so both sides describe the same house.

Boiler or heat pump, and when we offer quotes

• Boiler vs heat pump: the 15 year total is the installed cost plus 15 years of energy plus servicing of £100 a year for either system, at today's prices with no discounting. The heat pump is shown on the standard electricity rate and on an 18.7p heat pump tariff. The gas standing charge of about £108 a year is left out of both, because you only save it if you cap the gas supply. The verdict follows the heat pump tariff case, which is how most owners run one.
• Next steps: quote requests are paused at present, so each calculator points to the official installer registers instead. The heat pump calculator suggests getting installer quotes only where a heat pump saves more than £60 a year and pays back within 25 years on a heat pump tariff, after the grant. The boiler comparison suggests them only where the heat pump is cheaper over 15 years on a heat pump tariff, and the solar calculator only where the panels pay for themselves with years of their life left.

Insulation savings

Every insulation saving is measured, not modelled. The government compares the gas use of homes before and after a measure is fitted against similar homes that had nothing done, and publishes the median: 12.0% for cavity wall insulation across 2,191 homes, 17.3% for solid wall insulation across 2,411, and 3.2% for loft insulation across 1,486, including top-ups and room in roof insulation (NEED Impact of Measures 2026, Table 1). We apply those percentages to the home's own fuel use from the heat model and price them at the fuel it uses. When several measures are combined, each saving is taken from what is left after the ones before it.

Measured savings are lower than the modelled figures most calculators use, because many homes were under-heated before and take part of the gain as warmth rather than a lower bill. A home heated the same way before and after can save more than we show.

Retrofit plan

The retrofit plan combines the two. It lists the insulation a home may need, cheapest payback first, applies each measured saving in turn, then sizes and costs a heat pump for the home as it will be after that insulation. A whole-plan total counts the heat pump only where it lowers the bill. The same script runs the embeddable version, so the two cannot disagree.

Energy bills

The energy bill calculator and our bill guides add two parts. Heating is the heat demand model above, priced for the home's fuel. Everything else, the electricity for lighting, appliances and cooking, is the median a home with that many bedrooms used in England and Wales in 2024: 1,666 kWh for 1 bedroom, 2,137 kWh for 2, 2,659 kWh for 3, 3,371 kWh for 4 and 4,581 kWh for 5 or more (NEED consumption tables 2026, Table 4c). That median covers all homes, including the few heated by electricity, so it runs a little above what lights and appliances alone use. Both standing charges are added, the gas one only for homes on mains gas. One script, js/bills.js, produces every bill figure on the site, and a check compares the static tables with it before each release.

Solar panel calculations

Our solar calculator uses:

• Generation: the PVGIS (Photovoltaic Geographical Information System, European Commission Joint Research Centre) output for your postcode district, at the centre of its homes, less 7% for shading and dirt. Without a postcode, regional averages worked out the same way: 795 to 945 kWh per kW installed a year. Roof direction scales that output using PVGIS runs for London, Manchester and Edinburgh: south-east 0.95, south-west 0.93, east and west 0.80, a flat roof with tilted frames 0.90.
• Self-use ratio: read from the MCS MGD 003 Issue 2.0 look-up tables, which set self-consumption from occupancy, annual electricity use and system generation rather than a single figure. A 4 kW array on a medium user who is out half the day self-consumes about 19%, rising to about 52% with a 5 kWh battery.
• Installed cost: bands of £4,000 to £5,500 for 3 kW up to £8,500 to £10,000 for 6 kW, checked against the DESNZ solar PV cost data published 28 May 2026 (median about £1,550 to £1,600 per kW installed). Payback uses the middle of the band.
• Battery cost: £4,600 for a 5 kWh system, the Energy Saving Trust figure, added to the system cost when a battery is selected. A battery lasts about 10 to 12 years, so the 25 year figures include one replacement.
• Smart Export Guarantee: we use 12p per kWh, the Outgoing Octopus rate for Octopus electricity customers. Export tariffs open to customers of any supplier paid less, from 4.1p per kWh, and rates checked on 25 September 2026 ran from 4.1p to 17.5p depending on supplier and tariff.
• Panel degradation: 0.5% per year, which is the industry standard warranty assumption.

EPC calculations

Our EPC improvement planner is a simple points model, not an assessment. It estimates a score from the answers you give using fixed points for each feature, then adds typical points for each improvement. It follows the SAP bands (A 92 to 100 down to G 1 to 20) but does not run SAP, For a typical home of each age its estimate lands close to the English Housing Survey 2024 average rating (see EPC rating by house age), but we have not yet compared it home by home with lodged certificates. Treat the band as a starting point: only an accredited assessor can produce an EPC.

Grant eligibility

Our grant checker uses the published eligibility criteria from GOV.UK and the scheme administrators, and was last reviewed in September 2026. Schemes open, close and change, so always confirm eligibility with the scheme itself before you rely on it.

How we test the model

On 24 September 2026 we tested the model against the largest sets of measured data available for UK homes: installed costs from 30,590 grant funded heat pumps, efficiency from 428 monitored heat pumps, and insulation savings measured in thousands of homes. The model now uses those measured figures directly. Our data lists every source, when it was last updated and when we last checked it.

Limitations and disclaimers

Our calculators provide estimates based on national averages and published data. They are not substitutes for professional assessments. Key limitations include:

• Actual costs vary by location, installer, property condition, and access difficulty.
• Energy prices change quarterly and we cannot predict future prices.
• EPC improvements depend on the specific property and assessor methodology.
• Grant eligibility rules change and may vary by local authority area.
• All calculations run in your browser, and the figures you enter are not stored or sent anywhere. Copy link puts them into a link you can keep or share; our privacy policy explains what that means.

Always obtain professional quotes and assessments before proceeding with any installation.

Data licensing

This site contains public sector information licensed under the Open Government Licence v3.0. Energy price data from Ofgem. Energy efficiency data and Boiler Upgrade Scheme statistics from DESNZ. Heat pump performance from the Electrification of Heat trial (Energy Systems Catapult for DESNZ). Grant information from GOV.UK. Solar irradiance data from PVGIS (European Commission JRC).

Corrections and updates

If you spot an error in our calculations or data, please email hello@retrofitplanner.co.uk. We take accuracy seriously and will investigate and correct any verified errors promptly.