The answer
Can anyone guarantee my future bill?
A useful estimate starts with the home's annual heat demand, divides it by a realistic seasonal performance figure, then applies the electricity tariff. It should be a range, not a promise: weather, settings and prices will change.
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Start with the heat you will actually need
An old fuel bill is useful evidence, but it is not the whole calculation. The designer needs to estimate space-heating demand and hot water separately, using the size and fabric of the home, how it is used and the temperatures you want. If you currently keep rooms cold because your boiler is expensive to run, a new system that makes them comfortable may deliver more heat than the old bills suggest.
Then use a seasonal performance estimate for the proposed complete system. A brochure coefficient of performance measured at one outdoor and water temperature is not an annual average. The estimate should reflect the flow temperature your radiators need, hot-water production, defrost and any immersion or backup heating assumptions. Ask for these inputs in writing.
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Do the electricity arithmetic in front of you
Divide annual heat demand by the estimated seasonal performance to get approximate electricity use, then multiply by the relevant unit price. For example, if a home needs 12,000 kWh of delivered heat and the whole-system seasonal performance is assumed to be 3, it would use about 4,000 kWh of electricity. At an illustrative 25p per kWh, that is £1,000 for those units before any standing charge. At seasonal performance of 2.5, the same heat would need about 4,800 kWh and cost £1,200 at that same illustrative price.
This example is a calculation, not a forecast for your home or a statement of today's tariff. If an installer cannot explain the annual heat requirement, the performance assumption and the price applied, you cannot judge the running-cost headline. A time-of-use tariff may change the average price you actually pay, but only if your heat use and controls can take advantage of it without losing comfort.
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Compare it fairly with your current heating
Use the same rooms, temperatures, occupancy and hot-water demand on both sides. Record fuel unit rates and standing charges separately, because you may keep a gas supply for cooking or remove it entirely. Include servicing and any future price sensitivity if the comparison is being used to judge lifetime cost.
Ask for a range: a colder year, a lower seasonal performance and a higher electricity price. You can then see whether the proposed system is still affordable under less favourable assumptions. A free survey helps FLO establish the property-specific heat-loss and emitter design needed for that estimate; it should not be presented as a guaranteed saving.
Where this guidance comes from
Check the original guidance.
These primary sources support the technical or scheme points above. Rules can change; follow the current source before making a commitment.
Apply this to your property
Estimate costs from your home's real heating needs.
FLO's free survey establishes the design basis needed for a property-specific running-cost estimate and shows the assumptions you can challenge.