Immersion Heater Running Cost Calculator
See exactly what your immersion heater costs to run – per hour, day, month and year
Last updated: June 2026
Immersion Heater Running Cost Calculator
Enter your immersion heater's element rating and how long it runs to see the cost per hour, day, week, month and year, plus an Economy 7 off-peak comparison.
What this immersion heater cost calculator does
An immersion heater is an electric element that sits inside your hot water cylinder and heats the water directly, much like a giant kettle. Because it draws power straight from the mains rather than from gas, it can be one of the more expensive ways to heat water – so knowing the real cost matters. This calculator turns three simple inputs (your element's power rating in kilowatts, how many hours a day it runs, and your electricity unit rate) into a clear breakdown of what your immersion heater costs per hour, per day, per week, per month and per year.
It is built for UK households without a combi boiler, anyone on a hot-water cylinder with an immersion as their main or backup water heating, and renters or owners trying to cut winter energy bills. The default electricity rate is set to the Ofgem price cap of 26.11p per kWh (1 July to 30 September 2026, direct debit average for England, Scotland and Wales), but you can type in your own tariff rate for an exact figure. It also shows an Economy 7 off-peak comparison so you can see how much a timer and a cheaper night rate could save.
How the calculation works
The maths behind immersion heater running costs is straightforward:
- Energy used (kWh) = power rating in kW × hours running. A 3 kW element running for 1 hour uses 3 kWh.
- Cost = kWh × (unit rate in pence ÷ 100). At 26.11p/kWh, 3 kWh costs about £0.78.
The calculator multiplies your daily cost out across the days per week you use it, then scales weekly cost to a month (weekly × 52 ÷ 12) and a year (weekly × 52). The Economy 7 box re-prices the same annual kWh at your off-peak rate to estimate the saving from heating water overnight on a timer.
Worked example
Suppose you have a standard 3 kW immersion heater that runs for 2.5 hours a day, 7 days a week, on the price-cap rate of 26.11p/kWh:
- Energy per day = 3 kW × 2.5 hrs = 7.5 kWh
- Cost per day = 7.5 × (26.11 ÷ 100) = £1.96
- Cost per week = £1.96 × 7 = £13.71
- Cost per year = £13.71 × 52 = £712.80 (2,730 kWh)
Heating that same 2,730 kWh on a 15p off-peak Economy 7 rate would cost about £409.50 a year – a potential saving of roughly £303 if you can shift the heating to night-time using a timer.
Frequently asked questions
How much does a 3kW immersion heater cost to run per hour?
A 3 kW immersion heater uses 3 kWh of electricity per hour. At the Ofgem price-cap rate of 26.11p/kWh (1 July to 30 September 2026), that is about £0.78 per hour. A 6 kW element would cost roughly £1.57 per hour while it is actively heating.
Is it cheaper to leave the immersion heater on all the time or use a timer?
Using a timer is almost always cheaper. A well-insulated cylinder loses heat slowly, so for most households heating water once or twice a day on a timer uses less electricity than leaving the element on permanently, where the thermostat keeps re-firing to top up heat loss. On Economy 7, a timer set to the overnight cheap window can cut costs substantially – use the off-peak box above to estimate your saving.
Why does the immersion heater not run for the full time I switch it on?
The element has a thermostat that switches it off once the water reaches the set temperature (typically 60°C), then back on as the water cools. So a heater "on" for 3 hours may only draw full power for part of that time. The calculator estimates cost based on the hours you enter as actively heating; if you only switch it on for short top-ups, enter a smaller "hours per day" figure.
What electricity rate should I enter?
The default is the current Ofgem price-cap unit rate of 26.11p/kWh for July to September 2026. For an exact result, check your latest energy bill or smart meter app and enter your own unit rate. If you are on Economy 7, enter your daytime (peak) rate in the main rate box and your night-time rate in the off-peak box.
Source: Electricity unit rate from the Ofgem energy price cap unit rates and standing charges (price cap 1 July to 30 September 2026: 26.11p/kWh electricity, direct debit). Economy 7 off-peak rate is an editable typical figure and varies by supplier and region.