Show the formula and working
This immersion heater cost calculator works out how much electricity an immersion heater uses to heat a hot water cylinder, how long it takes and what it costs per heat up, per day and per year, for homes in the UK, Ireland and the rest of Europe. Enter the element power, cylinder size, water temperatures and your unit rate to see the result in pounds or euros.
Quick answer: An immersion heater cost calculator finds the energy to heat the water, then multiplies it by your unit rate. Heating a 120 litre cylinder from 10 °C to 60 °C takes about 6.97 kWh, so a 3 kW immersion runs for about 2 hours 19 minutes and costs about £1.83 at 26.32p per kWh.
What Is an Immersion Heater Cost Calculator?
An immersion heater cost calculator is a tool that estimates the electricity needed to heat stored hot water with an electric element, and the cost of that electricity. In plain words: litres of water x 4.18 x temperature rise in °C, divided by 3,600, gives the kWh needed. Divide the kWh by the element’s kW for the heating time, and multiply the kWh by your unit rate for the cost.
The figure 4.18 is the energy in kilojoules needed to warm 1 kg of water by 1 °C. The Wikipedia specific heat capacity article gives 4,185.5 J per kg per kelvin for liquid water at 15 °C, and a litre of water weighs about 1 kg. An immersion element turns all its electricity into heat in the water, so the only extra energy is the heat the cylinder loses while it stays hot, which you can add as a standing heat loss.
How Do You Use the Immersion Heater Cost Calculator?
- Enter the Immersion heater power in kW. Most UK elements are 3 kW, and the rating is stamped on the element head.
- Enter the Cylinder size in litres from the label on the tank.
- Set the Cold water temperature and the Thermostat setting. 10 °C and 60 °C are sensible starting points.
- Set Share of the tank heated each time. Use 100% for a full heat from cold, or a lower figure for topping up after a shower.
- Enter the Heat ups per day and, if you know it, the Standing heat loss in watts from the cylinder label.
- Check the Electricity unit rate. Use your night rate if the immersion runs on Economy 7, and switch to euros if you pay in cents.
- Read the cost per day, the cost per heat up and the heating time, then open “Show the formula and working” to check each step.
How Much Does It Cost to Run an Immersion Heater for an Hour?
A 3 kW immersion heater costs about 79p for each hour the element is on, at 26.32p per kWh. That is the average electricity unit rate under the Ofgem price cap for October to December 2026 for Direct Debit customers in England, Scotland and Wales. The thermostat switches the element off once the water reaches the set temperature, so it does not draw 3 kW all the time it is switched on.
How Long Does an Immersion Heater Take to Heat a Tank?
A 3 kW immersion heater takes roughly 1 hour 45 minutes to 4 hours to heat a typical UK cylinder fully from cold to 60 °C, depending on its size. The table shows the time and cost for common sizes with mains water at 10 °C and a unit rate of 26.32p per kWh.
| Cylinder size | Energy (10 to 60 °C) | Time at 3 kW | Cost per full heat | Cost per year (one full heat a day) |
|---|---|---|---|---|
| 90 L | 5.22 kWh | 1 h 44 min | £1.38 | £501.96 |
| 120 L | 6.97 kWh | 2 h 19 min | £1.83 | £669.27 |
| 150 L | 8.71 kWh | 2 h 54 min | £2.29 | £836.59 |
| 180 L | 10.45 kWh | 3 h 29 min | £2.75 | £1,003.91 |
| 210 L | 12.19 kWh | 4 h 4 min | £3.21 | £1,171.23 |
Most days you will not heat the whole tank from cold. If only the water used for a couple of showers needs reheating, the cost and time fall in proportion, which is what the share field models.
What Temperature Should an Immersion Heater Be Set To?
Set the immersion thermostat to 60 °C. The Health and Safety Executive’s legionella guidance says hot water should be stored at 60 °C or more to kill legionella bacteria. Turning it lower saves a little energy but raises that risk, and turning it much higher wastes energy and increases the risk of scalding, so a thermostatic mixing valve is worth considering where young children or older people live.
Is It Cheaper to Leave the Immersion On or Use a Timer?
Heating the water only when you need it is usually cheaper, because a hot cylinder loses heat all the time it is kept hot and the element has to replace that loss. A timer set to heat the tank an hour or two before you need it, ideally on cheaper off peak electricity, avoids paying to keep water hot overnight or while you are out. A well lagged modern cylinder loses less, so the saving is smaller.
If you have an Economy 7 meter, compare day and night costs with the Economy 7 calculator. For homes heating water with a boiler instead, the gas bill calculator helps you compare. Any change to the immersion’s wiring, switch or element must be done by a qualified electrician, because a 3 kW element needs its own circuit.
Worked Example: A 120 Litre Cylinder
Example: a 120 litre cylinder has a 3 kW immersion heater. Mains water is 10 °C, the thermostat is set to 60 °C and a full tank is heated once a day. The unit rate is 26.32p per kWh and Standing heat loss stays at 0.
- Temperature rise: 60 °C minus 10 °C = 50 °C
- Energy: 120 L x 4.18 x 50 / 3,600 = 6.97 kWh
- Time to heat: 6.97 kWh / 3 kW = 2 hours 19 minutes
- Cost per heat up and per day: 6.97 kWh x 26.32p = £1.83
- Cost per year: £1.83 x 365 = £669.27
If only 40% of the tank is reheated each day, the cost falls to about 73.3p a day and the heating time to about 56 minutes. A household in Ireland with the same cylinder and an example rate of 35c per kWh would pay about €2.44 for a full heat.
Immersion Heater Cost Calculator FAQs
How many kWh does an immersion heater use?
A 3 kW immersion uses 3 kWh for every hour the element is on. Heating a full 120 litre tank from 10 °C to 60 °C takes about 6.97 kWh, and a 210 litre tank about 12.19 kWh.
How much does it cost to heat water with an immersion heater per day?
One full heat of a 120 litre cylinder costs about £1.83 a day at 26.32p per kWh. Most households reheat only part of the tank, so their daily cost is often lower. Enter your own share and heat ups for a closer figure.
Is an immersion heater cheaper than gas for hot water?
Usually not on a standard tariff, because a kWh of mains gas costs much less than a kWh of electricity under the UK price cap. An immersion can be competitive on a cheap off peak rate or when it uses spare solar electricity.
Does a dual immersion heater cost more to run?
No. A dual immersion has a short element for topping up the top of the tank and a long element for heating the whole tank. Using the short one for small amounts of hot water can cut costs, because less water is heated.
How do I know if my immersion heater is on?
Most immersion switches have a red neon light that glows when the switch is on, and some timers show a light while the element is heating. A smart meter display will also show a jump of about 3 kW while the element heats.
Why does my immersion take longer than the calculator says?
Scale on the element, a faulty thermostat, colder winter mains water or a larger cylinder than you thought can all lengthen heating time. If the water never gets hot, ask a qualified electrician or plumber to check the element and thermostat.
Checked October 2026. Sources: Energy price cap unit rates and standing charges (Ofgem), Legionnaires’ disease: things to consider (HSE), Specific heat capacity (Wikipedia).