Calculator Methodology

Formula overview

Most calculators estimate annual energy use and cost by combining demand assumptions with fuel-price and efficiency assumptions.

• Estimated heat demand (kWh/year) x fuel unit price = annual running cost estimate.
• Heat pump electricity use is estimated as heat demand / SCOP.
• Boiler or legacy-system fuel use is estimated from demand and system efficiency assumptions.
• Indicative savings are estimated as current annual cost minus projected annual cost.

Inputs used

Inputs vary by tool but can include:

• Home size, heating demand proxy, and current fuel type.
• Electricity, gas, and oil price assumptions.
• Heat pump SCOP/COP assumptions and existing system efficiency assumptions.
• Indicative installation cost ranges and potential grant values.

Assumptions and interpretation

Results rely on generalized assumptions to provide comparable planning estimates. Actual project outcomes can differ due to occupancy, weather, control settings, property fabric, and final system design.

SCOP/COP explanation

COP (Coefficient of Performance) reflects instantaneous efficiency under specific conditions. SCOP (Seasonal COP) is a seasonal average and is generally more suitable for yearly planning estimates.

Energy price assumptions

Electricity, gas, and oil assumptions are simplified and can become outdated as tariffs change. Standing charges, regional differences, contract terms, and time-of-use structures can materially affect real costs.

Payback limitations

Simple payback outputs are directional only. They typically do not fully capture financing costs, maintenance, repair risk, comfort gains/losses, future fuel volatility, or wider home upgrade interactions.

Heat loss and sizing limitations

Sizing outputs are early-stage estimates. They are not a substitute for a professional room-by-room heat-loss calculation, emitter assessment, or installer-led design process.

Grant limitations

Grant values, eligibility rules, and application pathways vary by location and can change over time. Always verify current scheme terms with official sources before decisions.

Important disclaimer

Calculator results are educational estimates and are not financial, legal, engineering, or installation advice. A professional survey is required before committing to major home energy works.