How Much Does an Immersion Heater Cost to Run?

Updated July 2026 · Emergency Plumber Chelsea

How to work out your own running cost. Find the rating on the element's terminal cover or in the cylinder paperwork; almost all household immersions are 3kW. Look up the unit rate on your electricity bill, in pence per kWh. Multiply the two together and you have the cost of one hour of heating. If you are on Economy 7 or another time-of-use tariff, use the cheaper night rate for any heating the timer does overnight and the day rate for any boost during the day. That single sum is more reliable than any national average, because unit rates change and every home's tariff is different.

How much energy a tank of hot water takes. Heating water is simple physics: it takes about 1.16 watt-hours to raise one litre by one degree. Heating a typical 120-litre cylinder from cold mains at around 10°C to 60°C therefore needs roughly 7kWh, which a 3kW element delivers in a little over two hours. In practice you rarely heat a full tank from cold. After a couple of showers and some washing up, the element only has to reheat the water that was drawn off, so a normal day's reheat is often a fraction of that. Larger cylinders and hotter thermostat settings raise the figure; smaller households and shorter showers lower it.

Why the cylinder's insulation matters as much as the element. Hot water that is not used still cools down, and the immersion tops it back up. A modern cylinder with factory-applied foam insulation loses heat slowly. An old copper cylinder with a loose red jacket, a missing jacket or long runs of bare hot pipe in an airing cupboard loses far more, and you pay for that loss every day whether you run a tap or not. If your airing cupboard is noticeably warm, that warmth is electricity you are paying for. Lagging the first metre or two of pipe from the cylinder and fitting a proper jacket are among the cheapest ways to cut running costs; our pipe lagging guide covers which pipes to insulate. If you are not sure how your immersion is switched, our guide to what an immersion heater is explains single and dual elements and the bath and sink switch.

Is it cheaper to leave the immersion on all the time? Usually not. Leaving it switched on at the wall means the thermostat keeps the whole tank hot around the clock, including all the hours nobody needs hot water, so you pay to replace standing heat losses day and night. A timer that heats the water shortly before you need it, or overnight on a cheap tariff, almost always costs less. The exception is a very well insulated modern cylinder in a busy household, where the difference narrows. What costs the most is the forgotten boost switch or an immersion left on as a backup to a boiler that is already heating the same water.

Immersion heater or gas boiler: which heats water more cheaply? Electricity costs several times more per unit than gas in the UK, and a modern condensing boiler converts most of the gas it burns into heat. So in a home with a working gas boiler and a cylinder coil, heating hot water on the boiler is normally cheaper than heating it on the immersion. The immersion is best kept as a backup for when the boiler is off or being repaired. Homes without gas, and many Chelsea flats on all-electric systems, rely on the immersion as the main heat source, and there a good timer, a cheap overnight tariff and a well insulated cylinder make the biggest difference. For the wider comparison of heating fuels, see our guide to heat pumps versus gas boilers.

Signs your immersion is costing more than it should. A bill that has crept up without any change in how you use hot water can point to a thermostat that is not switching the element off, so it runs continuously and the water comes out scalding. A hissing, knocking or rumbling cylinder often means limescale has built up around the element, which slows heat transfer and makes it run longer; hard London water makes this common, and our hard water guide explains the effect. Water that is only lukewarm despite long heating times can mean a failed element in a dual-element cylinder or a thermostat set too low. A timer that has lost its settings after a power cut can leave the immersion heating at the wrong time of day.

Setting the thermostat sensibly. Most immersion thermostats sit under the element cover and are adjustable. Around 60°C is a sensible setting for a stored hot water cylinder, hot enough to control the risk of legionella bacteria growing in the tank without heating water far hotter than you need. Turning it much lower to save money is not recommended for a stored cylinder. Turning it higher costs more and raises the scalding risk. Adjusting the thermostat means removing the cover over live terminals, so switch off and isolate the supply first, or ask a qualified person to do it.

Dual-element cylinders and Economy 7. Many older cylinders in Chelsea flats have two elements or a dual element: a long one that heats the whole tank overnight on the cheap rate and a short one near the top for a quick daytime boost of a smaller volume. Used properly, the overnight element does most of the work at the lower rate and the boost is only for occasional top-ups. Used badly, with the boost left on or the timer set to daytime hours, the same cylinder can cost a lot more to run. It is worth checking your timer settings match your tariff's cheap hours, which vary by supplier. If you are on a time-of-use tariff or have solar panels, a smart cylinder such as a Mixergy can heat only the water you need at the cheapest times. Flats with a Gledhill Pulsacoil thermal store heat on Economy 7 in the same way; our guide to Pulsacoil problems covers what to check when one stops giving hot water.

When running costs point to a repair or replacement. If the element is scaled up, the thermostat has failed or the cylinder is losing heat through a poor jacket, fixing the specific fault is usually far cheaper than living with the extra running cost. An element or thermostat replacement is a routine job. A cylinder that is badly scaled, poorly insulated and old may be worth replacing with a modern factory-insulated one, and our guide to what size hot water cylinder you need helps you avoid heating more water than your household uses.

Getting it checked. If your hot water is too hot, too slow to heat, noisy or not heating at all, our guide to an immersion heater not working covers the safe checks first. For a repair, a new element or thermostat, or advice on whether to keep the immersion or rely on the boiler, Emergency Plumber Chelsea covers SW3, SW10, Knightsbridge, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours. Call 020 3002 6667 or see our plumbing repairs service.

Quick tips

• Cost per hour = element kW (usually 3) x your unit rate in pence per kWh. Check both on the element cover and your bill.
• Use a timer rather than leaving the immersion on at the wall; standing heat losses are what you pay for around the clock.
• If you have a working gas boiler with a cylinder coil, heat water on the boiler and keep the immersion as a backup.
• Fit a proper cylinder jacket and lag the first stretch of hot pipe; a warm airing cupboard is paid-for heat escaping.
• Keep a stored cylinder at around 60°C; much lower is not recommended, much higher wastes money and risks scalding.

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