Heat Pump Savings Calculator
Compare your annual running cost on gas vs an air source heat pump — on the standard Ofgem cap, Octopus Cosy, and Octopus Go tariffs. With 15-year cumulative savings.
Your home
Assumes constant prices & SCOP. Real prices may rise — but gas typically rises faster than electricity (per OBR forecasts).
How heat pump savings actually work
The headline question is simple: does it cost less to run an air source heat pump than a gas boiler?
The answer is "almost always yes, but the size of the saving depends on three things":
- Your heat pump's SCOP — Seasonal Coefficient of Performance. SCOP 3.0 means every 1 kWh of electricity becomes 3 kWh of heat. SCOP 4.0 means 4 kWh. Well-installed units hit 3.0–3.5; poor installs drop to 2.4.
- Your electricity tariff — Octopus Cosy and Go are 25–35% cheaper than the standard Ofgem cap during heat-pump-favourable hours.
- The gas/electricity price ratio — currently 3.77× (26.35p ÷ 6.99p). Heat pumps win whenever their SCOP exceeds that ratio. SCOP 3.8+ on the standard cap means cheaper than gas. SCOP 2.8+ on Cosy means cheaper than gas.
Why Octopus Cosy changes the maths
Octopus Cosy is a time-of-use tariff designed specifically for heat pump households. It offers three rate tiers:
- Cheap rate (12.20p): 4–7am, 1–4pm, 10pm–midnight
- Standard rate (24.40p): most of the rest of the day
- Peak rate (32.50p): 4–7pm — heat pump should pre-heat the home before this window
Set your heat pump controls to load up the buffer tank and hot water cylinder during cheap hours, and you'll average around 19p/kWh annually. That's the killer ratio: 19p ÷ 6.99p = 2.72× — easily beaten by a SCOP-3.0 heat pump.
15-year savings, not just year-one
The default view is year-1 saving. The bigger story is what those savings compound to over the heat pump's 15-year design life:
| Scenario | Year-1 saving | 15-year saving |
|---|---|---|
| SCOP 3.0 ASHP on standard cap | ~£75 | ~£1,100 |
| SCOP 3.0 ASHP on Octopus Cosy | ~£280 | ~£4,200 |
| SCOP 4.2 GSHP on Octopus Cosy | ~£450 | ~£6,750 |
| SCOP 2.6 ASHP (cold/poor install) on cap | −£100 | −£1,500 |
The bottom row is the warning: a poorly-installed pump in a cold area on the standard tariff costs more than gas. This is why installer skill matters more than brand, and why a smart tariff matters more than the pump model.
What about gas price inflation?
The Office for Budget Responsibility projects UK gas prices to rise 4-6% per year over the next decade, while wholesale electricity is forecast to fall 2-3% per year as renewables share grows. Both effects widen heat pump savings over time — the year-15 saving is typically 60-80% larger than year-1 once price drift is factored in.
This calculator uses constant prices for simplicity. If you want a stress test, run the gas price at 9p (a +30% scenario) and your heat pump savings double.