If you've been stuck on the fence about a heat pump because you can't quite tell whether it'll genuinely save you money to run, this page is the maths laid out plainly. The core tension is simple: a modern condensing gas boiler turns 1 kWh of gas into roughly 0.92 kWh of useful heat. A modern air-source heat pump turns 1 kWh of electricity into 3.5–4.2 kWh of useful heat. That 4× efficiency multiplier is what makes a heat pump viable even though electricity costs about 4× more per kWh than gas in 2026. Whether the multiplier actually works in your home depends on two things: how well the pump is designed, and what tariff you end up on.
2026 unit prices — what each fuel actually costs
| Fuel | Standard tariff | Best fixed | Heat-pump optimised |
|---|---|---|---|
| Gas (per kWh) | 7.2p | 6.4p | n/a |
| Electricity (per kWh) | 28p | 24p | 15.5p (Octopus Cosy day blend) |
| Heating oil (per litre) | 74p (~7.5p/kWh equiv) | 69p | n/a |
Cost per kWh of useful heat — the comparison that matters
| System | Efficiency | Tariff | Cost per kWh of heat |
|---|---|---|---|
| Gas combi boiler (new) | 92% | 7.2p/kWh | 7.8p |
| ASHP (typical) | SCOP 3.7 | 28p/kWh standard | 7.6p |
| ASHP (typical) | SCOP 3.7 | 15.5p/kWh Cosy blend | 4.2p |
| ASHP (well designed) | SCOP 4.2 | 15.5p/kWh Cosy blend | 3.7p |
| Oil boiler (new) | 92% | 7.5p/kWh | 8.2p |
| Old gas boiler (15yr) | 78% | 7.2p/kWh | 9.2p |
Three real homes, three real annual bills
Take a 3-bed semi in Chelmsford with 12,000 kWh of heating demand and 3,000 kWh of hot water demand — a very typical UK load. Here's what each setup actually costs over a year:
- New gas combi: 15,000 kWh ÷ 0.92 × 7.2p = £1,174 a year for heat and hot water combined.
- ASHP on a standard 28p tariff: 15,000 kWh ÷ 3.7 × 28p = £1,135 a year. Roughly the same as gas.
- ASHP on Octopus Cosy (blended ~17p): 15,000 kWh ÷ 3.7 × 17p = £689 a year. Saving of £485 a year against gas.
- ASHP on Cosy with a 5kW solar PV offsetting ~30% of heat: £482 a year. Saving of £692 a year against gas.
The 2026 heat-pump tariff lineup
- Octopus Cosy — three off-peak windows a day. Designed around heat pumps with a hot-water cylinder pre-heat.
- Octopus Intelligent Octopus Heating — pump-aware control, charges your hot water at the cheapest price automatically.
- Eon Next Drive v2 — broad EV-and-pump tariff with a 6.7p overnight window.
- British Gas Hive Heat Pump Tariff — bundled with Hive controls, 11.5p flat with no off-peak dip.
- OVO Heat Pump Plus — 17p flat, lower than standard, but no time-of-use saving to chase.
Carbon — the case even when bills are flat
Even before the tariff savings kick in, the carbon picture is clear. A gas boiler emits 0.184 kg CO₂ per kWh of heat. The 2025 UK grid average puts an ASHP at SCOP 3.7 at 0.054 kg CO₂ per kWh — roughly 70% lower. By 2030, as grid decarbonisation continues, that gap widens to about 85%. For households on a flat tariff where the bill saving is marginal, the carbon case alone tips the answer.
When gas still wins (for now)
A poorly insulated solid-wall property with single-glazed windows and undersized rads will struggle on a heat pump unless the fabric is fixed first. In those cases the short-term maths can still favour gas — though the right order of operations is always 'fix the fabric, then revisit the heating.' A 12-month BUS-plus-ECO4 stack routinely makes the heat pump viable on the same property once insulation is in. We always quote the fabric work first; you don't want a heat pump cycling into a cold house.
For a real-world cost comparison based on your actual heat demand and your specific tariff options, book a free home energy survey online or ring 0800 229 4094. We'll model the bills for you on the day. No cost. No obligation. No pressure.
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