Whether a heat pump is cheaper to run than your gas boiler comes down to three numbers: the price of gas, the price of electricity, and how efficiently each system turns that fuel into heat. Everything else is noise.
The efficiency gap
A modern condensing gas boiler runs at roughly 85-90% efficiency in real homes. For every 1 kWh of gas you buy, you get about 0.88 kWh of heat.
A well-installed air source heat pump has a SCOP (seasonal coefficient of performance) of 3.0 to 4.0. For every 1 kWh of electricity, you get 3-4 kWh of heat. A poorly installed one, running hot radiators that were never resized, can drop to 2.2-2.5 — which is where most disappointing heat pump stories come from.
The price gap
Electricity costs roughly 3.5-4 times more per kWh than gas on a standard UK tariff. That is the whole problem: the efficiency advantage and the price disadvantage very nearly cancel out.
| System | Efficiency | Effective cost per kWh of heat |
|---|---|---|
| Gas boiler | 88% | Gas unit rate divided by 0.88 |
| Heat pump (SCOP 3.0) | 300% | Electricity rate divided by 3.0 |
| Heat pump (SCOP 3.8) | 380% | Electricity rate divided by 3.8 |
On a standard single-rate tariff, a heat pump at SCOP 3.0 lands within a few percent of a gas boiler — sometimes slightly cheaper, sometimes slightly dearer. At SCOP 3.8, it is clearly cheaper.
Where heat pumps pull ahead: the tariff
This is the part most comparisons skip. Several UK suppliers now offer heat pump specific electricity tariffs with a heavily discounted rate for heating. On one of those, the effective cost per kWh of heat drops well below gas — typically a 20-40% saving against a gas boiler on the same house.
To use one you generally need:
- A heat pump with a separate or identifiable heating circuit
- A smart meter
- The installer to register the system with your supplier
Ask about this at quote stage, not after installation.
A worked example
Take a 3-bed semi using 12,000 kWh of gas a year for heating and hot water.
- Gas boiler: 12,000 kWh of gas at the standard unit rate, plus the gas standing charge.
- Heat pump at SCOP 3.5: that same house needs about 10,500 kWh of delivered heat, so roughly 3,000 kWh of electricity — and the gas standing charge disappears entirely if you remove gas from the property.
Losing the gas standing charge is worth around £100-£120 a year on its own, and it is routinely left out of comparisons.
What actually determines your running cost
- Flow temperature. A heat pump running at 45°C is far more efficient than one running at 55°C. That means correctly sized radiators, not the ones your boiler was happy with.
- Insulation. Loft and cavity wall insulation cut demand for both systems, but they matter more with a heat pump because they let you run cooler.
- Controls. Heat pumps like steady, low, continuous operation. Running one like a boiler — blasting it on twice a day — wrecks the SCOP.
- Hot water strategy. Heating the cylinder overnight on cheap electricity is a meaningful saving; a daily legionella cycle at 60°C using the immersion is a meaningful cost.
The honest summary
If you get a properly designed system with resized emitters, a low flow temperature and a heat pump tariff, expect to spend less than you do on gas. If you get a like-for-like swap on existing radiators at 55°C on a standard tariff, expect running costs to be broadly similar or slightly higher.
The design of the installation matters more than the brand of the heat pump. That is why comparing several quotes — and specifically comparing the design flow temperature and predicted SCOP each installer states — is the single most useful thing you can do.
Read next: what a heat pump quote should include and heat pump grants in the UK.