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What happens to your bills if gas prices fall 30 %?
The stress test nobody selling heat pumps runs. If gas drops 30 % and electricity stays put, what happens to your running cost and your payback?
Every heat pump calculation is a bet on the ratio between two prices you do not control. It is worth knowing what happens if that bet goes against you, and the answer is less dramatic than either side of the argument suggests.
The stress test
Take a house using 16,000 kWh of delivered heat, a 90 % condensing boiler, and a heat pump at SCOP 3.0, with electricity held at the July 2026 cap:
| Gas price | Ratio | Break-even SCOP | Boiler cost | Heat pump cost | Difference |
|---|---|---|---|---|---|
| 7.33p (cap, today) | 3.56 | 3.20 | £1,303 | £1,393 | -£89 |
| 6.23p (−15 %) | 4.19 | 3.77 | £1,108 | £1,393 | -£285 |
| 5.13p (−30 %) | 5.09 | 4.58 | £912 | £1,393 | -£481 |
| 9.53p (+30 %) | 2.74 | 2.47 | £1,694 | £1,393 | £302 |
Assumptions behind these figures
- 16,000 kWh delivered heat, boiler seasonal efficiency 90 %, heat pump SCOP 3.0
- Electricity held at £0.2611/kWh throughout — this is the assumption doing the most work
- Standing charges excluded on both sides, so the comparison is unit-rate only
- Gas price scenarios are illustrative, not forecasts
Reading it honestly
A 30 % gas price fall would make a SCOP-3.0 heat pump clearly more expensive to run than a boiler at these electricity prices, and would push out the payback on a system already installed. That is a real risk and it should be stated plainly rather than argued away.
Two things temper it. First, UK electricity and gas prices are coupled, because gas plant sets the marginal electricity price for much of the year — a large sustained gas fall tends to drag electricity down with it, which holds the ratio closer to constant than the table implies. Second, the policy costs loaded onto electricity rather than gas are the subject of active debate; any rebalancing moves the ratio in the heat pump's favour independently of wholesale prices.
What reduces your exposure
- A lower flow temperature. A SCOP of 3.9 survives a 30 % gas fall. Efficiency is the only hedge you own outright.
- A heat pump tariff. It shifts your electricity price below the cap, which shifts the whole table.
- Not over-capitalising. A £13,000 install with a £7,500 grant has a net cost of £5,500, which recovers under most scenarios. A £22,000 install does not.
Test the scenario you are actually worried about in the break-even calculator — it takes both prices as inputs for exactly this reason.
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Questions people ask
What if gas prices fall?
A 30 % fall in gas prices with electricity unchanged would make a SCOP-3.0 heat pump more expensive to run than a 90 % efficient gas boiler. A SCOP of 3.9, or a heat pump electricity tariff, both survive that scenario.