Heating on low all day vs on a timer
Compare the daily and monthly cost of leaving your heating on all day against a timer, for a gas boiler, heat pump or electric heaters, at the Ofgem price cap rates, with how cold the house gets in between.
- The timer saves
- £0.14 a day
- £4.35 over 30 days, 3% less than heating all day
- Heating all day at 20°C
- £4.68 a day
- £140.34 over 30 days. 54.0 kWh of heat from 58.7 kWh of gas
- On a timer, off in between
- £4.53 a day
- £135.99 over 30 days. 50.1 kWh of heat from 56.9 kWh of gas
- Coldest on the timer
- 16.6°C
- At 14:53. The heating comes on at 04:36 and 14:53 to be back to 20°C by 06:30 and 17:00
The timer is cheaper here, by £0.14 a day (3%), even though we assume the boiler runs a little less efficiently on it. A boiler only puts back the heat that leaked out, and a cooler house leaks less.
These are costs just for the heating, as an illustration: no standing charge, no hot water, and no free heat from people, cooking and the sun, which would lower both. The house loses 150 W for every degree it’s warmer than outside and holds 3.75 kWh per degree, so with the heating off it loses about two thirds of its warmth above outside in 25 hours. It’s 5°C outside all day and night. We step through a day minute by minute, after enough days that each day ends where it started. The heating gives up to 8 kW, and on the timer it comes on early to be warm when each period starts: at full output. The boiler is 92% efficient heating all day and 88% on the timer. Condensing boilers do a few per cent better with cooler water, so running low and slow only helps if the flow temperature is turned down. Gas is 7.97p a kWh. Treating the house as one lump makes it slow to warm up, so the early start here is longer than a real home needs. Real rooms feel warm sooner, and a model that lets the air warm faster than the walls would probably show bigger timer savings, so trust the direction more than the pennies.
Prices are Ofgem’s price cap averages for Direct Debit in England, Scotland and Wales from 1 October to 31 December 2026: gas 7.97p a kWh including 5% VAT, and electricity 26.32p a kWh, with no VAT on electricity from 1 October 2026 to 31 March 2027. Your own rate depends on your region and tariff. Northern Ireland isn’t covered by the price cap or the VAT cut, so put in the unit rate from your bill.
What goes in
Choose your heating (a gas boiler, a heat pump or electric heaters, though not storage heaters on a night tariff) and your kind of home: well insulated, typical, leaky, or your own heat loss figure from a heat pump survey. Then the outside temperature, how warm you want it, when the timer is on, and whether the heating goes off or down to a lower temperature (a set-back) in between. The unit rates start at the price cap and you can change them.
What the result means
The calculator works out what each way costs over a day and over 30 days, just for the heating, and how cold the house gets on the timer. By default the timer comes on early enough to be warm when each period starts, so both ways keep you equally warm while you’re up and home.
With a gas boiler, a timer usually costs less, because a house leaks heat in proportion to the gap between inside and outside, and a cooler house leaks less. In a typical semi on a 5°C day the model puts heating all day at 20°C at £4.68 and a timer from 06:30 to 08:30 and 17:00 to 23:00 at £4.53. The gap is bigger in a leaky home and about nothing in a very well insulated one.
With a heat pump, steady running or a small set-back usually wins, because warming the house back up needs hotter water, which makes the heat pump less efficient. The calculator warms a heat pump up gently, as a weather-compensated controller would, and in the typical semi a timer or a 2 or 3 degree set-back comes out about level with heating all day. The model keeps the outside temperature the same day and night, so it can’t show that a set-back also spares the heat pump from working hardest in the colder night air. The Energy Saving Trust advises keeping a heat pump on with a set-back when you’re out or asleep, and Heat Geek suggests two or three degrees.
Treat the pounds as an illustration. The house is modelled as one lump, which is slow to warm up; real rooms warm faster, and a model where the air warms faster than the walls would probably show bigger timer savings.
The figures behind it
Prices are Ofgem’s price cap averages for Direct Debit in England, Scotland and Wales from 1 October to 31 December 2026: gas 7.97p a kWh including 5% VAT, electricity 26.32p a kWh with no VAT on electricity from 1 October 2026 to 31 March 2027. Standing charges are left out, as they’re the same whether you use a timer or leave it on. Northern Ireland isn’t covered by the cap or the VAT cut, so put in the rates from your bill.
At those prices, a kWh of heat from a 90% efficient gas boiler costs 8.9p (7.97 ÷ 0.9). From a heat pump it’s 6.6p at a COP of 4, 8.8p at 3 and 10.5p at 2.5.