How much does it cost to run an immersion heater for an hour?

Multiply the element rating by your electricity unit rate. A standard 3kW element uses 3kWh per hour of heating, so at 25p per kWh an hour costs 75p, and at 30p it costs 90p. That is the cost while the element is actually heating. Once the water reaches the thermostat setting the element switches off, so an hour with the switch on usually costs less than that.

How much electricity does an immersion heater use?

A 3kW immersion uses 3kWh for every hour it runs. Heating a 150 litre cylinder from cold to 60 degrees takes just under 8kWh, a 210 litre cylinder about 11kWh. Day to day you are usually replacing part of a tank, plus one to two kWh of standing losses on a modern cylinder, more on an old jacketed one.

Is it cheaper to leave the immersion on or switch it off?

Switch it off or put it on a timer. Leaving it on keeps the whole tank at temperature around the clock, so you pay to replace heat lost through the insulation even when nobody needs hot water. On a well insulated modern cylinder the difference is modest. On an old cylinder with a thin jacket it is significant.

Is it cheaper to heat water with the immersion or the gas boiler?

The gas boiler, in almost every case. Gas costs much less per kWh than electricity, so a tank heated through the boiler coil costs a fraction of the same tank heated by the element. The immersion is best treated as a backup for when the boiler is off or broken.

Why has my immersion heater suddenly started costing more?

The usual causes are a scaled element running longer to deliver the same heat, a thermostat stuck on so the water overheats, a timer that no longer matches your tariff, or the boiler side failing so the immersion is doing work the boiler used to do. Scalding water or an element that never seems to switch off both need an engineer.

Can I turn the immersion thermostat down to save money?

Not below 60 degrees. Stored hot water should be held at 60 degrees to control Legionella bacteria. If the water at the taps is too hot, the answer is a thermostatic blending valve on the outlet, not a lower storage temperature.

How Much Does an Immersion Heater Cost to Run?

An immersion heater costs its power rating multiplied by your electricity unit rate for every hour the element is actually heating. Almost every domestic element is 3kW, so it uses 3kWh an hour: at a unit rate of 25p that is 75p an hour, and heating a typical 150 litre cylinder from cold takes close to 8kWh, so roughly £2 a tank at that rate. The switch being on is not the same as the element running. Once the water reaches the thermostat setting the element switches itself off, and from then on you only pay to replace the heat the cylinder loses and the hot water you draw. Your own figure depends on your tariff, the size of the tank and the state of the cylinder, and all three are easy to check.

How to work out what your immersion costs per hour

You need two numbers. The first is the element rating, which is stamped on the head of the immersion under the round plastic cap, usually as 3kW and occasionally as 2.75kW or 3.6kW. The second is the unit rate on your electricity bill, in pence per kWh. Multiply one by the other and you have the cost of one hour of the element heating flat out.

A 3kW element at 25p per kWh costs 75p an hour. At 30p it is 90p. At a night rate a good deal lower than your day rate, the same hour costs proportionally less, which is the whole argument for running an immersion overnight on Economy 7 or a similar tariff.

That hourly figure is the ceiling, not the typical cost. The element only draws power while the rod thermostat is calling for heat. On a well insulated cylinder that has already been heated, the element runs for a few minutes at a time to top the tank back up, not for the full hour the switch has been on.

Cost per hour = element rating in kW x unit rate in pence per kWh
3kW at 25p per kWh: 75p for each hour of heating
3kW at 30p per kWh: 90p for each hour of heating
The rating is on the immersion head; the unit rate is on your bill

What it costs to heat a full cylinder

The honest way to think about immersion running cost is per tank rather than per hour, because what you are paying for is the energy it takes to lift a volume of water through a temperature rise. That is simple physics: about 1.16 watt hours per litre for every degree. Bringing mains water at around 15 degrees up to the 60 degrees a cylinder should be stored at is a 45 degree rise.

For a 150 litre cylinder that works out at just under 8kWh, which is why the same tank takes around two and a half to three hours on a 3kW element. A 120 litre cylinder needs a little over 6kWh. A 210 litre unvented cylinder needs about 11kWh. Multiply those by your unit rate and you have the cost of a full reheat from cold, before any allowance for heat escaping through the insulation while it works.

In practice you rarely reheat a whole tank from cold. A morning of showers might draw half the cylinder, and the immersion only has to replace that half. So a household running on the immersion alone typically pays for something between a half and a full tank a day, plus the standing losses covered below.

120 litre cylinder, 15 to 60 degrees: a little over 6kWh
150 litre cylinder, 15 to 60 degrees: just under 8kWh
180 litre cylinder, 15 to 60 degrees: about 9.4kWh
210 litre cylinder, 15 to 60 degrees: about 11kWh

Does leaving the immersion on all the time cost more?

It costs more than it needs to, but not as much as people fear. With the switch permanently on, the thermostat keeps the whole cylinder at temperature around the clock. You pay to replace every bit of heat that leaks through the insulation, including overnight and while the house is empty, and you pay for it at whatever rate applies at that moment.

How much that adds depends almost entirely on the cylinder. Modern factory insulated cylinders publish a standing heat loss figure in kWh per 24 hours on the data label or product sheet, and for a typical domestic unit it sits in the region of one to two kWh a day. An older copper cylinder in a loose fitting jacket loses a great deal more. A simple check: put a flat hand on the cylinder a couple of hours after it finished heating. It should feel barely warm. If it feels properly warm, or the airing cupboard is hot, that heat is on your bill.

The better habit is to run the immersion for the time it takes to satisfy the thermostat before you need the water, and switch it off or put it on a timer. The guide to how long does hot water take to heat up covers the timings for one shower against a full tank.

Is the immersion or the boiler cheaper for hot water?

If you have a working gas boiler heating the cylinder through a coil, the boiler is almost always the cheaper way to heat the water. Gas costs much less per kWh than electricity, typically somewhere around a quarter to a third of the price, and even after allowing for the boiler not being perfectly efficient, a tank heated by gas costs a fraction of the same tank heated by the element.

That is why the immersion in most homes with a boiler is a standby. It is there for the day the boiler breaks down, not for daily use, and the explainer on what is an immersion heater covers why the two sit side by side. If you have got into the habit of using the immersion because the boiler side never seems to heat the cylinder properly, that is a fault worth fixing rather than a running cost to accept. A cylinder that only gets hot on the immersion usually has a motorised valve, a cylinder thermostat or a programmer problem on the boiler side, which is the same set of causes behind no hot water but heating works.

Homes with no gas are different. A flat with a direct cylinder, heated only by immersion elements, has no cheaper alternative, and the running cost comes down to the tariff and the timer. If you are not sure which type you have, the difference between a direct vs indirect hot water cylinder is worth two minutes of looking at the pipework.

Economy 7 and off peak tariffs

Cylinders built to be heated electrically usually have two elements: a long one entering near the bottom to heat the whole tank, and a short one near the top to heat the upper third. The intended pattern is that the bottom element charges the whole cylinder overnight on the cheap rate, and the top element is used as a short daytime boost if the tank runs low.

This only saves money if the timer and the elements are doing what you think. Common problems we find: the timer still set to the old clock after a power cut, the bottom element dead so the overnight charge only heats the top of the tank, or the boost left permanently on so the day element does all the work at the day rate. Any of those turns a cheap tariff into an expensive one, and if your tariff has changed since the cylinder was fitted, check the timer matches the new cheap hours.

Why your immersion may be costing more than it should

When an immersion bill looks wrong, the cause is usually a fault rather than the tariff. The common ones are worth knowing, because each has a clear sign.

Scale on the element is the big one in London. Hard water leaves a crust of limescale on the hottest surface in the cylinder, which is the element. The crust insulates it, the element runs longer to deliver the same heat, and eventually it fails. A stuck thermostat is the other expensive fault: if the rod thermostat fails closed, the element never switches off, the water gets far hotter than 60 degrees and you pay for every extra degree until the safety cut-out trips. Scalding water from the taps with the immersion on is the sign, and it is covered on our page about a hot water cylinder overheating.

Element heating for hours and the water still only lukewarm: scale, or a failing element
Water far too hot at the tap: the thermostat is not switching off
Hot water runs out after one shower on Economy 7: the bottom element has probably failed
Warm airing cupboard, warm cylinder hours after heating: poor insulation
Boiler working but you still need the immersion for hot water: a boiler side control fault

Never switch an immersion on when the cylinder has been drained or might be part empty. The element is designed to be surrounded by water and will burn itself out in seconds if run dry.

Cutting the running cost without creating a health risk

Do not turn the thermostat down below 60 degrees to save money. Stored water held at a lukewarm temperature for days is the condition Legionella bacteria need to multiply, and a few pence a day is not worth that trade. Our guide on what temperature should a hot water cylinder be set at explains the setting and why it matters.

What does work: run the immersion on a timer rather than permanently, put the heating window on the cheap rate if you have one, fit a proper factory insulated cylinder or at least a well fitted jacket if yours is old, lag the first metre or so of hot pipe leaving the cylinder, and use the boiler rather than the immersion wherever a boiler exists. If the element is scaled or the thermostat is suspect, replacing them costs less than a winter of running a failing element.

When to get the immersion looked at

If the immersion is taking far longer than the figures above, the water is scalding, the element trips the electrics, or you are relying on it because the boiler side will not heat the cylinder, it is worth having an engineer test it. An immersion heater repair starts with the element, the rod thermostat and the cut-out tested on a meter, and on a vented cylinder the parts are standard. Unvented cylinders often take make specific elements, and anything on the sealed side of an unvented unit needs a G3 qualified plumber.

Call 020 4652 5111 and tell us the make of the cylinder, whether you have a boiler, and what the immersion is or is not doing. Our rates are on the pricing page, agreed before we attend.

Questions we get asked

How much does it cost to run an immersion heater for an hour?: Multiply the element rating by your electricity unit rate. A standard 3kW element uses 3kWh per hour of heating, so at 25p per kWh an hour costs 75p, and at 30p it costs 90p. That is the cost while the element is actually heating. Once the water reaches the thermostat setting the element switches off, so an hour with the switch on usually costs less than that.
How much electricity does an immersion heater use?: A 3kW immersion uses 3kWh for every hour it runs. Heating a 150 litre cylinder from cold to 60 degrees takes just under 8kWh, a 210 litre cylinder about 11kWh. Day to day you are usually replacing part of a tank, plus one to two kWh of standing losses on a modern cylinder, more on an old jacketed one.
Is it cheaper to leave the immersion on or switch it off?: Switch it off or put it on a timer. Leaving it on keeps the whole tank at temperature around the clock, so you pay to replace heat lost through the insulation even when nobody needs hot water. On a well insulated modern cylinder the difference is modest. On an old cylinder with a thin jacket it is significant.
Is it cheaper to heat water with the immersion or the gas boiler?: The gas boiler, in almost every case. Gas costs much less per kWh than electricity, so a tank heated through the boiler coil costs a fraction of the same tank heated by the element. The immersion is best treated as a backup for when the boiler is off or broken.
Why has my immersion heater suddenly started costing more?: The usual causes are a scaled element running longer to deliver the same heat, a thermostat stuck on so the water overheats, a timer that no longer matches your tariff, or the boiler side failing so the immersion is doing work the boiler used to do. Scalding water or an element that never seems to switch off both need an engineer.
Can I turn the immersion thermostat down to save money?: Not below 60 degrees. Stored hot water should be held at 60 degrees to control Legionella bacteria. If the water at the taps is too hot, the answer is a thermostatic blending valve on the outlet, not a lower storage temperature.