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Heat pump vs gas boiler: a 2003-2011 detached house
A worked example with this building's own numbers, not an average. Everything below is calculated from the assumptions listed with it, and you can change any of them in the calculator.
Assumptions behind these figures
- House: detached house, 2003-2011, 140 m²
- Space-heat demand as found 75 kWh/m²/yr
- SCOP 3.6 @ 40 °C
- Assumed existing boiler efficiency 90 %
- Ofgem energy price cap, 1 July – 30 September 2026 (direct debit, average). Oil/LPG/coal are editable estimates, not primary-sourced.
- Capital cost £13,000
- Simple payback: no discount rate, no fuel-price inflation, no maintenance difference. Figures are calculations, not quotes.
Fabric and demand
| Treated floor area | 140 m² |
| Space-heat demand as found | 75 kWh/m²/yr |
| Annual space heating | 12,600 kWh |
| Hot water | 2,200 kWh |
| Total delivered heat | 14,800 kWh |
| Implied heat loss | 7.0 kW |
| Assumed design flow temperature | 40 °C |
| Assumed SCOP | 3.6 |
| Assumed existing boiler efficiency | 90 % |
How these numbers were produced
- Delivered heat needed: 12,600 kWh (space) + 2,200 kWh (hot water) = 14,800 kWh
- Fuel energy at 90 % seasonal efficiency: 14,800 ÷ 0.90 = 16,444 kWh
- Current annual cost: 16,444 kWh × £0.0733 + £315 standing charges = £1,520
- Heat pump electricity: 14,800 kWh ÷ SCOP 3.6 = 4,111 kWh
- Heat pump annual cost: 4,111 kWh × £0.2611 + £209 standing charge = £1,282
- Running-cost difference: £1,520 − £1,282 = £238
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £238 |
| Simple payback | 23.1 years |
Boiler Upgrade Scheme, £7,500 (air source)
What actually decides it in this house
At 40 °C the SCOP is about 3.6, which is break-even against gas at the current cap and a clear win against anything else. This is the fabric level where a heat pump stops being an argument.
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