The answer
Can an installer tell you exactly what your future heating bill will be?
An installer can estimate running costs, but cannot guarantee your future bill. Ask what weather, electricity price and indoor temperature the estimate assumes, and how the cost changes if those differ.
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Your old bill is only a starting point
What you spent on gas or oil depends on fuel prices and how efficiently the old system worked. First estimate how much heat the home needs, then how much electricity a heat pump would use to provide it.
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Use an estimate for the whole year
A heat pump's best test result is not its year-round performance. Ask what efficiency the forecast assumes through winter and summer, including hot water and any backup heater.
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Check the energy prices in the estimate
Ask which gas, oil and electricity prices were used and whether standing charges still apply. A cheap overnight tariff should not be assumed for every hour of heating unless the system can actually use it.
Why should a running-cost estimate show a range?
Start with the heat needed for rooms and hot water. Estimate the system's seasonal efficiency at the proposed water temperatures, then calculate electricity use. Price that electricity using a stated tariff and compare it with the existing fuel on the same comfort and occupancy basis. Standing charges, immersion use and backup heat should not disappear silently from the arithmetic.
Ask to see a colder-year or higher-tariff case. A household that chooses warmer rooms or longer heating hours after installation may use more heat than the historic fuel record suggests. That is a comfort benefit, not necessarily a failed heat pump, but it belongs in the estimate.
What to check next
Your next steps
Where we checked the facts
Check the original information.
These official pages back up the rules and technical details above. They can change, so check them again before spending money or starting work.
Apply this to your own property
Could a heat pump work at your property?
Every property is different. A free survey checks heat loss, hot water and radiators to see whether a heat pump may fit before a system is priced.