Answer first
For gas, take annual kWh used for heating and hot water, multiply by a reasonable estimate of the boiler’s seasonal efficiency, then divide the useful heat by the proposed heat pump’s seasonal efficiency. Multiply the resulting electricity by your tariff.
For oil or LPG, convert litres into kWh before applying boiler efficiency. For direct electric heating, the delivered heat is much closer to the metered electricity used, but storage losses, hot water and rooms heated still need separating.
Illustrative annual gas input used in the worked example
View sourceIllustrative old-boiler seasonal efficiency—not a measured value for your boiler
View sourceIllustrative heat-pump SPF used to show the method
View sourceDiscuss heat-pump costs and savings for your home
Share your postcode, current heating and the bill question you want answered. FLO can explain what needs measuring before any saving figure is treated as realistic.
Collect a full year before calculating
Use at least 12 months so winter and summer are both represented. Meter reads are better than estimated bills. Note unusual occupancy, an extension, a broken boiler, portable heaters or a winter that was not representative.
Separate non-heating uses where possible. A gas cooker is usually small, but a gas fire, pool, annex or commercial use can distort the total. If the boiler provides hot water, keep that demand in the comparison only if the proposed heat pump will provide it too.
- Twelve months of actual meter reads or delivery records
- Fuel used by other appliances
- How many rooms and hours were heated
- Any supplementary electric heaters
- Household or building changes planned after installation
Worked gas example
Suppose annual gas input is 15,000kWh and 85% becomes useful space heat and hot water. That gives 12,750kWh of useful heat. Dividing by an illustrative heat-pump SPF of 3 gives 4,250kWh of electricity.
At 26.11p/kWh, that electricity costs about £1,110. The same 15,000kWh of gas at 7.33p/kWh costs about £1,100 before standing charges. This close result illustrates why performance, tariff and removal of the gas standing charge can change the comparison.
Keep in mind: The 85% and SPF 3 assumptions are examples. Substitute a survey-based heat demand and a documented performance estimate before making a decision.
Oil and LPG need an extra conversion
Delivery invoices record litres, not useful heat. Convert the fuel volume with an appropriate calorific value, then apply a seasonal boiler-efficiency assumption. Tank levels and deliveries rarely align perfectly with a calendar year, so use a sufficiently long period and record the starting and ending tank position.
Older off-gas systems can have different standing losses and servicing costs from gas. A fair comparison should include the costs that actually disappear, continue or are added—not just the unit price of fuel.
Historic use can understate the comfort you want
Some households ration heat, leave rooms cold or use fires and plug-in heaters. Their fuel history reflects that behaviour. If the new design aims to maintain more rooms at a steady temperature, useful heat demand may rise even though comfort improves.
Tell the surveyor which rooms are difficult, which are rarely heated and what indoor temperatures you expect. A heat-loss calculation should remain the primary design basis; historic use is a cross-check that can reveal implausible assumptions.
Turn the estimate into a range
Calculate a lower, central and upper case using realistic efficiency and tariff assumptions. This shows which variable matters most and avoids presenting a single pound figure as certainty.
Ask the installer to reconcile its forecast with your history. A large unexplained gap may point to an incorrect floor area, temperature assumption, fuel conversion or hot-water allowance.
- Low, central and high seasonal efficiency
- Current tariff plus a sensitivity for future price changes
- Present comfort versus intended comfort
- Gas standing charge removed or retained
- Maintenance and any finance costs shown separately
What to read next
Related answers and services
Questions people also ask
Can I use the pound value on my bill instead of kWh?+
No. Prices and tariffs change. Use energy units first, then apply the tariff relevant to each comparison.
What if my bills are estimated?+
Request actual meter history from your supplier or collect readings over time. Estimated bills can make a useful-heat cross-check misleading.
Does a low gas bill mean a heat pump is unsuitable?+
No. It may mean the home has low demand, low occupancy or under-heating. Suitability still depends on the property, system design and practical installation.
Evidence used for this answer
Figures are explained in context and links go to the independent source. Prices, tariffs, scheme rules and standards can change, so check the source again before making a financial or technical decision.
- Ofgem: energy price cap unit ratesPublished electricity and gas unit-rate benchmarks. Rates vary by region, payment method, tariff and price-cap period.
- Energy Systems Catapult: heat-pump performance analysisMeasured seasonal performance and cold-weather operation from the Electrification of Heat demonstration project.
- Energy Saving Trust: air source heat pumpsIndependent guidance on installation costs, running costs, suitability, radiators and household savings.
- MCS heat-pump design standardTechnical requirements for heat loss, equipment selection, domestic hot water and system design.