Heat Pump vs Gas Boiler Calculator

The honest maths behind heat pumps

A heat pump does not create heat the way a boiler does — it moves it, harvesting warmth from the outside air (or ground) and concentrating it for your radiators and hot water. That is why its efficiency is quoted not as a percentage but as a SCOP: a Seasonal Coefficient of Performance of 3.2 means that over a whole heating season, each kilowatt-hour of electricity becomes 3.2 kWh of heat. A gas boiler at 85% efficiency turns each kWh of gas into 0.85 kWh of heat. On efficiency alone the heat pump wins by nearly four to one.

But your bills are not priced in efficiency — they are priced in pence per kWh, and here the UK has an awkward truth. Gas costs roughly 6p per kWh while electricity costs roughly 24p: a 4-to-1 price ratio that almost exactly cancels the heat pump's efficiency advantage. At the default inputs in this calculator (SCOP 3.2, the standard price ratio), a £900 gas bill becomes a £956 heat pump bill — you pay more to run the cleaner system. Many calculators hide this behind optimistic defaults. We surface it, because a decision made on hidden numbers is not a decision at all.

The way out of the paradox is on both sides of the fraction: cheaper electricity, better SCOP, or both. Dedicated heat-pump tariffs (Octopus Cosy, OVO Heat Pump Plus and similar) price off-peak electricity far below the standard 24p; enter 18p in the electricity field above and watch the verdict change. Meanwhile a well-designed system — correctly sized, with low flow temperatures and decent radiators — achieves SCOP 3.5–4.0 rather than 3.2, which is worth hundreds of pounds a year. Both levers are in your control; the gas/electricity price ratio is the government's.

SCOP: the number that decides everything

SCOP is not a fixed property of the box on your wall — it is a property of the whole system, and the installer matters more than the brand. The three biggest drivers are flow temperature, sizing, and controls. Heat pumps are happiest producing lukewarm water (35–45°C) continuously; force one to run at 60°C+ to feed undersized radiators and the SCOP collapses toward 2.5, at which point even cheap electricity struggles to beat gas. Oversized units short-cycle (constantly switching on and off), which also murders efficiency. And weather compensation — the controller adjusting flow temperature to the outdoor temperature automatically — is the single cheapest SCOP booster available.

What does this mean for quotes? Ask every installer for their designed SCOP and the design flow temperature, in writing. A quote that cannot state a SCOP is a quote from someone who has not done the heat-loss calculation — walk away. The Microgeneration Certification Scheme (MCS) requires a room-by-room heat loss survey for a compliant install; if your survey took twenty minutes and a tape measure, it was theatre. Good design takes hours and produces a document you can keep.

Radiators: the upgrade nobody mentions in the advert

Here is the conversation most heat pump marketing skips: your radiators were sized for 70°C water from a boiler, and a heat pump wants to run at 40°C. At lower temperatures each radiator gives off less heat, so many homes need larger radiators — typically upsizing 30–50% of them, or adding fan-assisted radiators in key rooms. Budget £1,500–£3,000 for radiator work on top of the heat pump quote if your installer flags it, and be suspicious of any quote that declares your existing radiators "fine" without a room-by-room calculation. Underfloor heating is the ideal partner (huge surface area, very low flow temperature) but is rarely worth retrofitting into an existing home purely for the heat pump. Take our heat pump readiness quiz to see where your home stands before spending anything.

The Boiler Upgrade Scheme: £7,500 off

The Boiler Upgrade Scheme (BUS) gives England and Wales homeowners £7,500 toward an air-source or ground-source heat pump replacing fossil-fuel heating (oil, gas, LPG, or direct electric). Your installer applies for the voucher and discounts your bill — you should never be asked to pay full price and "claim it back". Key conditions: you must own the property (including landlords), it must have a valid EPC with no outstanding loft or cavity wall insulation recommendations, the install must be MCS-certified, and new-builds are excluded. The grant has been extended and the budget is real, but apply through your installer early — vouchers are issued per installation and the paperwork takes weeks. Scotland has its own, more generous scheme (Home Energy Scotland: up to £7,500 grant plus £7,500 interest-free loan) — our grant finder covers all four nations.

When a heat pump does NOT pay

Say it plainly: do not install a heat pump in a leaky home. If your walls are uninsulated, your loft is bare, and your windows are single-glazed, a heat pump will run constantly at high flow temperatures, achieve a miserable SCOP, and cost you more than the boiler it replaced — while the installer blames your house. The correct order is fabric first: insulation, draught-proofing, and an honest audit of where you stand. Also think twice if you have no outdoor space for the unit (they need airflow and make a low hum — check permitted development noise limits with neighbours close by), if your electricity supply needs an expensive upgrade, or if you plan to move within a few years: the payback needs you to stay put.

Carbon: the argument that survives the maths

Even when the money is marginal, the carbon case is not. Grid electricity keeps getting cleaner as renewables grow, so a heat pump installed today gets greener every year it runs; a gas boiler locks in 0.21 kg of CO2 per kWh for its entire 15-year life. At the default inputs above, switching saves roughly 2.3 tonnes of CO2 a year — about a quarter of the average UK household's total footprint. If your motivation is climate rather than cash, the calculator's CO2 figure is the number to watch, and pairing the pump with solar panels (daytime electricity for a daytime-running pump) is the strongest combination a UK home can currently buy.

Running costs in practice: the tariff trick

The single highest-value action after installation is switching to a heat-pump-friendly tariff. Standard variable tariffs charge a flat ~24p all day; heat pump tariffs offer cheap overnight and shoulder-period rates, and because a well-insulated home with a pump can pre-heat during cheap hours, real-world savings of £200–£400 a year versus the standard tariff are common. Set a calendar reminder to re-check tariffs every year — the market moves, and loyalty is priced accordingly. Also consider: every degree you lower your flow temperature saves roughly 2–3% on running costs, so the annual service should include a review of the weather-compensation curve, not just a filter clean.