How to cut heating bills: one home, four calculators

This guide follows one house through the four Home energy calculators, step by step. It is a two-storey mid-terrace of about 85 m², built between 1965 and 1982, with unfilled cavity walls, 100 mm of loft insulation, an uninsulated solid floor, double glazing from before 2002 and a condensing gas boiler. The heating and heat loss calculators open on this house by default, and the links below open each step with the figures used here, so you can follow every figure and then swap in your own details. Prices are the Ofgem cap for 1 October to 31 December 2026.

Step 1: what the heating costs now

Open the heating cost comparison. From the floor area and the age of the house, it estimates 7,583 kWh of heat a year, hot water included. The estimate is based on the median gas use of similar homes in England and Wales. On the condensing boiler that heat costs £828 a year, of which £108.33 is the gas standing charge.

Before going further, check this against your own statement. Take your yearly gas use in kWh and multiply it by 0.84, the boiler efficiency. If the result is very different from 7,583, choose “I know my yearly heat use” and type in your own number. Everything that follows scales with it.

Step 2: find out where the heat goes

Open the heat loss calculator. It already holds this house’s walls, loft, floor, windows and doors. Built up part by part, the house loses 8,312 kWh a year in an average UK winter. This is about 10% more than step 1’s estimate. The two use different methods, and the difference between them is a reminder that both are estimates.

What matters more is the split. The walls take 29% of the heat loss, the floor 21%, ventilation and draughts 20%, windows 17%, the loft 10% and the doors 4%. Two common jobs are already filled in under “after the work”: filling the cavity and topping up the loft to 270 mm. Together they cut the heat loss to 6,536 kWh. That saves 1,776 kWh of heat and about £168 a year in gas.

Most of that comes from the walls. Filling the cavity alone saves £120 a year; topping up the loft alone saves £48, because the first 100 mm already does most of the work. Enter the quotes you get in “Cost of the work” to see a payback time for each job. The calculator does not guess at prices.

Step 3: compare heating systems for the insulated house

Below its results, the heat loss calculator offers a link that carries the new heat demand into the heating comparison. It is the same as the heating comparison at 6,536 kWh. At that demand the condensing boiler costs £728 a year and an air-source heat pump £619. The calculator puts the saving at about £110 a year, the same as before the insulation; it works from the unrounded costs, which differ by £109.67. That is no coincidence. At current prices a kWh of heat costs 9.49p from the gas boiler and 9.47p from a heat pump with the default efficiency (SCOP 2.78). The saving is almost entirely the gas standing charge, which you only stop paying if you close the gas supply.

Two conclusions follow for this house. Insulation lowers the bill whatever heats the home, and it reduces the size of heat pump needed if you install one later. And whether a heat pump saves money here depends almost wholly on how efficiently it runs and on whether you give up gas entirely. At this demand, type 2.5 into the heat pump’s efficiency and the saving falls to £40; type 3.0 and it rises to £155.

Step 4: the electricity side

Open the electricity usage calculator. Its default list of thirteen rows adds up to 3,718 kWh a year, about £979 at 26.32p per kWh. Lighting (537 kWh) and standby (438 kWh) are the two largest rows. Both are worth checking in a real home: LED bulbs cut the first, and a night-time meter reading shows the second. The list includes a fridge, a fridge-freezer and a chest freezer, so remove what you do not have.

If the house moved to a heat pump after insulating, its electricity use would rise by about 2,351 kWh a year, the heat pump figure the heating comparison shows at 6,536 kWh. Gas would no longer be needed for heating.

Step 5: would Economy 7 help?

Open the Economy 7 calculator with this house’s 3,718 kWh. The night share is set to 29%, which is what you would get if you used electricity evenly round the clock. The standard tariff comes out cheaper by about £31 a year. The two totals on the page, £1,179 and £1,209, are rounded, which is why they differ by £30; the verdict uses the unrounded costs. Economy 7 pays off only once at least 33.7% of use falls in the seven night hours. That break-even stays at 33.7% even with a heat pump’s extra electricity, because it depends on the gap between day and night rates, not on how much you use.

A gas-heated house like this one rarely gets above a third of its use at night without storage heaters, an immersion heater on a timer or an electric car charged overnight. If you already have a two-rate meter, divide the night units by the total over the same months to find your real share.

What the five steps add up to

For this house at today’s prices, the two insulation jobs save about £168 a year in gas, the heat pump about £110 a year (almost all of it the gas standing charge), and Economy 7 would cost about £31 a year more, not less. Your own quotes decide whether any of the work pays back; the calculators only show the yearly side. A house with an older boiler, solid walls, oil heating or storage heaters will give quite different figures, so run the steps with your own.

Before you rely on the numbers

Every figure here is an estimate from average data, not a quote and not a promise of savings. The heat figures assume an average UK year at a 15.5 °C base and a home heated evenly. The appliance figures come from homes measured in 2010–11. Delivered fuels and Economy 7 rates are marked as estimates on the pages. Ofgem notes that there is no VAT on electricity from 1 October 2026 to 31 March 2027. When the cap changes on 1 January 2027, open the calculators again with the new rates from your supplier.

Three things on your own paperwork will improve every step: your yearly gas and electricity use in kWh from your annual statement, the unit rates and standing charges on the same statement, and your Energy Performance Certificate, which records wall type, loft insulation and glazing. In England and Wales, the government’s Boiler Upgrade Scheme offers grants towards heat pumps and biomass boilers. Check what applies to your home on that page, and enter the cost after any grant when you work out a payback.