A heat pump beats a gas boiler on running cost when its COP exceeds the electricity-to-gas price ratio multiplied by the boiler's efficiency. With a 90%-efficient boiler that means a break-even COP of 2.7 at a 3:1 price ratio, 3.6 at 4:1 and 4.5 at 5:1. At the EU-average ratio of roughly 2.4 (Eurostat, late 2025), any heat pump running above COP 2.2 heats for less.
How do you calculate the break-even COP?
Break-even COP = (electricity price ÷ gas price) × boiler efficiency. That single formula settles the running-cost argument, because it compares fuels per kWh of delivered heat rather than per kWh of fuel. A boiler at 90% efficiency delivers heat at gas price ÷ 0.9; a heat pump delivers it at electricity price ÷ COP. Setting the two equal and rearranging gives the formula. Worked example: electricity at 3.5 times the price of gas and a 90% boiler gives 3.5 × 0.9 = 3.15 — a heat pump averaging better than COP 3.15 across the season is cheaper to run, and one averaging below it is dearer. Note that the seasonal average (SCOP or measured SPF) is what counts, not the headline figure from a mild test day.
What is the break-even COP at different price ratios?
The table shows the break-even seasonal COP against a 90%-efficient condensing boiler, with an 80% column for older non-condensing boilers:
| Electricity : gas price ratio | Break-even COP (90% boiler) | Break-even COP (80% boiler) |
|---|---|---|
| 3 : 1 | 2.7 | 2.4 |
| 4 : 1 | 3.6 | 3.2 |
| 5 : 1 | 4.5 | 4.0 |
Two readings follow. First, against an old non-condensing boiler the bar drops noticeably — many replacements beat an 80% boiler even in unfavourable markets. Second, the ratio is everything: Eurostat put the EU-average household electricity-to-gas ratio at roughly 2.4 in late 2025, where the break-even COP is only about 2.2 — a target almost any properly installed unit clears. High-ratio markets are the hard cases, which is why dedicated heat pump tariffs matter; see which electricity tariff is cheapest for a heat pump.
Is the commonly cited COP of 3.4 correct?
The figure of about 3.4, commonly cited in the UK running-cost debate, is simply the formula evaluated at one snapshot of UK standard-tariff prices — a ratio near 3.8 with a 90% boiler. It is not a law of physics, and it moves whenever tariffs move: switch to a heat pump tariff or time-of-use rate and the ratio falls, dragging the break-even COP down with it. Treat any single-number claim as shorthand for "at these prices, on this tariff". The full 2026 market comparison, including Eurostat price data by country, is in heat pump vs gas boiler running costs.
Can a real heat pump actually reach the break-even COP?
Yes — a well-designed air-to-water system reaches a seasonal COP of 3.5–4.0, comfortably above break-even at typical EU price ratios. The European market's growth reflects that arithmetic: heat pump sales rose 13% in 2025 to 2.88 million units (EHPA). Design quality decides which side of the line you land on: low flow temperatures, correct sizing and weather compensation push real installations toward 4, while oversized units, 55 °C flow settings and poor controls drag them below 3 — the failure modes catalogued in why is my heat pump COP so low. Manufacturer data marks the ceiling: Solimpeks' Varm Up Series is rated at COP up to 4.9 at the A7/W35 rating point, and the installation determines how much of that survives a real winter.
What does the formula leave out?
Standing charges, carbon pricing and capital cost sit outside the running-cost formula. Dropping the gas connection entirely removes the gas standing charge — a fixed saving the kWh comparison never shows. From 2028, ETS2 carbon pricing loads cost onto gas heating across the EU, lowering the effective ratio year by year, while the EU's stated policy direction is to shift levies off electricity. And grants change the capital side of the equation without touching the running side — see how the UK's Boiler Upgrade Scheme works. The honest conclusion: compute the break-even COP from your own tariff, compare it with the measured seasonal performance of a properly designed system, and ignore any argument that quotes prices without the ratio.
Frequently asked questions
What COP does a heat pump need to be cheaper than gas?
Multiply your electricity-to-gas price ratio by your boiler's efficiency. At a 3:1 ratio with a 90% boiler the answer is COP 2.7; at 4:1 it is 3.6. At the EU-average ratio of about 2.4, roughly COP 2.2 is enough.
Where does the often-quoted COP of 3.4 come from?
It is the break-even formula evaluated at one snapshot of UK standard-tariff prices — a ratio near 3.8 times 90% boiler efficiency. It is commonly cited but tariff-dependent: heat pump tariffs lower the ratio and the required COP with it.
Do real heat pumps achieve a seasonal COP above 3.5?
Well-designed air-to-water systems do, typically reaching 3.5–4.0 seasonally when run at low flow temperatures with weather compensation and correct sizing. Poorly designed installations can fall below 3, which is a design problem rather than a technology limit.
Does boiler efficiency really matter in the comparison?
Yes — it scales the break-even COP directly. Against an older non-condensing boiler at 80% efficiency, the threshold at a 4:1 price ratio drops from 3.6 to 3.2, so replacing an old boiler favours the heat pump more than the headline numbers suggest.