How much does an immersion heater cost to run?

How to work out the cost for your cylinder

Look at the element rating first. Most domestic immersion heaters are 3 kW; the rating is usually on the element head under the cover, or in the cylinder paperwork. Then find your electricity unit rate on your bill, in pence per kWh. If you are on an off-peak tariff such as Economy 7, you will have two rates.

The energy needed to heat water depends on how much water and how many degrees you are raising it. Roughly, every 100 litres raised by 10°C takes about 1.16 kWh. A 150-litre cylinder filled with mains water at around 10°C and heated to 60°C needs about 8.7 kWh. A 3 kW element delivers that in just under three hours.

In real use the cylinder is rarely heated from completely cold. After a shower or two, only part of the water has been replaced with cold, so the element runs for a shorter time. The cylinder also loses some heat through its insulation, and the element has to make that up as well.

Multiply the kWh figure by your own unit rate. The price cap rate is an average; your tariff, region and payment method all change it, and the cap is reviewed every quarter.

Is an immersion heater cheaper than gas?

Almost never, per litre of hot water. Under the October to December 2026 cap, Ofgem's average unit rates are 26.32p per kWh for electricity and 7.97p per kWh for gas. An immersion heater turns nearly all its electricity into heat in the water, while a gas boiler loses some energy up the flue. Even allowing for those boiler losses, heating the same cylinder with gas usually costs well under half as much as heating it with the immersion.

That is why, in a home with a working gas boiler and an indirect cylinder, the immersion is normally a backup for when the boiler is out of action. If your hot water only seems to work with the immersion switched on, that points to a fault on the boiler side, such as a zone valve, cylinder thermostat or programmer problem, rather than a reason to keep paying for electric hot water.

The picture changes if electricity is your only option, or if you have cheap electricity at certain times. On an off-peak tariff, a cylinder with a bottom element on a timer can heat the full volume overnight at the lower rate, with a top element used only for boosts. Homes with solar panels can divert surplus generation into the immersion through a diverter, so much of the hot water comes from electricity that would otherwise be exported.

Should you leave an immersion heater on all the time?

An immersion heater has its own thermostat, so leaving it on does not mean the element runs constantly. It switches on when the water cools below the set point and off again when it reaches it. But a cylinder kept hot all day and night keeps losing heat through its insulation and pipework, and the element keeps replacing that heat whether anyone uses the water or not.

For most homes, heating the water on a timer shortly before the times you use it costs less than keeping it hot around the clock. How much you save depends on how well the cylinder is insulated. A modern factory-insulated cylinder loses far less than an old copper cylinder with a loose jacket.

Do not turn the temperature down too far to save money. HSE guidance is that hot water should be stored at 60°C or above to control legionella bacteria. Setting the thermostat well below that is a false economy if it creates a health risk, particularly for older or vulnerable people.

If scalding at the taps is the worry, a thermostatic mixing valve at the bath or basin limits the outlet temperature while the cylinder stays at 60°C.

Faults that make an immersion cost more than it should

A sudden jump in electricity use, or water that is hotter than it should be, often means something is wrong rather than that the immersion is simply expensive. These are the problems we find most often.

• A failed immersion thermostat that does not switch the element off, leaving water scalding hot and the element running far longer than needed
• The immersion left on by accident alongside a working boiler, so both are heating the same cylinder
• Scale on the element in London's hard water, which slows heat transfer and lengthens heat-up times
• Poor or missing cylinder insulation and bare hot pipework in a cold airing cupboard or loft
• A dripping hot tap or a passing valve that keeps drawing hot water out of the cylinder
• A timer or switch that has failed in the on position, so the water is kept hot all day

If the water from the taps is scalding, steam is coming from the cylinder or its vent, or the tundish under an unvented cylinder is discharging hot water, switch the immersion off at the switch and get the cylinder checked. The safety controls may have been working hard to cope with a thermostat fault.

Ways to reduce the running cost

Use the boiler for everyday hot water if you have one, and keep the immersion as a backup. Put the immersion on a timer rather than a manual switch, and on an off-peak tariff make sure the timer matches your cheap-rate hours.

Check the cylinder insulation and lag the hot pipes nearest the cylinder. Our guide to hot water cylinder heat loss insulation explains what to look for and when a replacement cylinder makes more sense than improving the old one.

Match the heat-up time to how you use hot water. A cylinder that is too large heats water you never use; one that is too small forces extra boosts. Our guide to hot water cylinder recovery time explains how cylinder size, element rating and demand fit together.

In a hard water area, a scale reducer or softener protects the element as well as the rest of the system. Our article on whether a water softener is worth it compares the options.

Get your immersion or cylinder checked

Loyal Plumbers provides immersion heater repair and wider hot water repair across London and everywhere inside the M25. We replace failed elements and thermostats, fit timers, and check whether a boiler fault is making you rely on the immersion. Where the cylinder itself is the problem, we can advise on replacement.

Call us or send an enquiry with the type of cylinder you have, whether you have a gas boiler, what the immersion switch is set to and what has changed. If the immersion has stopped working altogether, our guide to an immersion heater not working covers the safe checks first.