The useful formula
Annual heating cost ≈ heat needed ÷ seasonal efficiency × electricity price
That means a confident running-cost promise is impossible without knowing the home’s heat demand, the designed seasonal performance and the tariff. A good proposal should show the assumptions rather than hide them.
The five biggest running-cost drivers
Heat demand
Insulation, draughts, floor area, indoor temperature and weather determine how much heat the home needs.
Seasonal efficiency
Performance changes through the year. The design should estimate seasonal efficiency, not rely on a best-case brochure number.
Electricity tariff
The unit rate and any heat-pump or time-of-use tariff can materially change the annual cost.
Flow temperature and controls
Lower flow temperatures and steady, correctly commissioned control usually support better efficiency.
The old heating system
Savings look very different when replacing direct electric, LPG, oil or a modern gas boiler.
Why “three times efficient” does not mean one-third of the bill
Energy Saving Trust explains that heat pumps can deliver more than three units of heat for each unit of electricity used. But electricity and fossil fuels have different unit prices, and a home’s previous system may also have different losses and standing charges.
The comparison should use annual heat delivered, realistic seasonal performance and the tariffs available to you. If gas is removed completely, avoiding the gas standing charge can also affect the result.
Questions to ask before accepting a savings estimate
- What annual space-heating and hot-water demand is assumed?
- What seasonal coefficient of performance is used?
- Which electricity tariff and unit price are assumed?
- What flow temperature is the system designed around?
- Does the comparison include standing charges and boiler efficiency?
- Are changes in comfort or heating hours being compared fairly?
Independent sources
Scheme rules and guidance can change. Check these sources before making a financial or planning decision.