Heat pump calculator: methodology

We aim to be radically transparent about how this calculator works. Every formula, default value, and data source is documented below. If you disagree with any of our assumptions, you can override them in the calculator inputs.

The calculation, step by step

1. Annual heat demand

We start with your annual gas usage in kWh (from your bill or annual statement) and convert that to heat actually delivered to your home:

heat demand (kWh) = gas usage × boiler efficiency

We default to 85% boiler efficiency. Real boilers vary 70-95%, but whole-season real-world efficiency averages 80-87% even for modern condensing boilers (cycling losses, hot water demand, oversizing). This figure is from the Energy Saving Trust's technical guidance.

2. Heat pump electricity needed

HP electricity (kWh) = heat demand ÷ SCOP

SCOP is the Seasonal Coefficient of Performance - how much heat your heat pump produces per unit of electricity, averaged across a year. We default to 3.0 conservatively. MCS-installed heat pumps typically achieve 3.0-3.5 in real UK homes; well-designed systems with low-flow temperatures achieve 3.5-4.0+.

3. Annual cost comparison

current cost = (gas usage × gas rate) + (gas standing charge × 365)
HP cost = (HP electricity × electricity rate) + standing charge contribution

Standing charge handling is nuanced. If you keep your gas supply, you continue paying the gas standing charge regardless. If you remove gas entirely (an option for full electrification), you save the gas standing charge - but only the actual gas standing charge, since you were already paying electricity standing for the rest of your home.

4. Payback period

net install cost = install cost - grant
payback years = net install cost ÷ annual saving

We don't apply discounting to future savings (i.e. we don't adjust for the time value of money or assume energy price inflation). These factors typically cancel out roughly and add complexity without much accuracy. Real-world payback could be 1-2 years better or worse depending on how rates evolve.

5. CO₂ savings

gas CO₂ = gas usage × 184 g/kWh
HP CO₂ = HP electricity × 124 g/kWh (UK grid average)
saving = gas CO₂ - HP CO₂

Gas combustion factor (184 gCO₂/kWh) from DESNZ greenhouse gas reporting conversion factors. UK grid factor (124 gCO₂/kWh) is the 2025 annual average from NESO. The grid is decarbonising rapidly, so actual lifetime CO₂ savings will be higher than this calculation shows.

Important things this calculator does not do

• Heat loss calculation:We don't size the heat pump for you. Use your gas usage as a proxy for heat demand, which works well for typical homes but not for poorly-insulated properties.
• Radiator sizing or flow temperature design: The SCOP you achieve depends heavily on system design. A heat pump pushed to run at high flow temperatures (because radiators are too small) will underperform its rated SCOP. Get a heat loss survey from an MCS installer for accurate sizing.
• Time-of-use tariff modelling:If you're on a half-hourly tariff like Octopus Cosy, the calculator uses an average rate. Real savings on Cosy depend on running the heat pump during off-peak windows.
• Insulation upgrades:Often the best decarbonisation investment is insulation before a heat pump. We'll add a dedicated insulation calculator in a future update.

Data sources

• Ofgem energy price cap (rates updated quarterly): ofgem.gov.uk
• Boiler Upgrade Scheme grant rules: gov.uk
• DESNZ greenhouse gas conversion factors: gov.uk
• NESO Carbon Intensity: carbonintensity.org.uk
• Energy Saving Trust - heat pump and boiler guidance: energysavingtrust.org.uk

Updates and changes

Tariff rates are updated daily from the Octopus Energy public API. Ofgem price cap rates are updated within a week of each quarterly announcement. Grant scheme rules are reviewed quarterly. Methodology changes are documented in the project's public commit history.

Found a problem with the calculation? It matters - please email feedback@[domain]and we'll respond within a few days.