An immersion heater is a large electric element that heats the water in a cylinder directly, like a giant kettle. It is reliable and useful as a backup, but because it heats with electricity rather than gas it is one of the dearer ways to make hot water. The cost is easy to work out, and it is just as easy to leave one quietly running up the bill, so it is worth knowing both what a tankful costs and the one trick that roughly halves it.
The short answer. A typical immersion heats a full cylinder using around 7 to 12 kWh depending on tank size, so at the standard electricity rate of about 26p a unit that is roughly 1.80 to 3.15 pounds a tankful. Heated overnight on an Economy 7 night rate it costs about half that. Rely on the immersion for all your hot water and a year can run to several hundred pounds at the standard rate, which is why a gas boiler, or off-peak heating, makes such a difference.
Why it costs what it does
A typical immersion element draws around three kilowatts and runs for an hour or more to bring a full tank up to temperature. The element itself is not inefficient; nearly all the electricity it draws ends up as heat in the water. The expense is the fuel: electricity costs about 26p a unit on the standard rate, while a gas boiler heats the same water at roughly 8p a unit of heat, as the hot water savings guide explains. So heating water by immersion costs around three times what the same hot water costs from gas. That is the whole reason an immersion has a reputation as a pricey way to make hot water, and why it matters how, and when, you run one.
What a tankful costs
The energy to heat a cylinder depends on its size, because you are warming a fixed volume of water from cold mains temperature up to storage heat. The table below works out a full reheat for the common cylinder sizes, both at the standard price-cap rate of about 26p per kWh, current in mid 2026, and at an example Economy 7 night rate of around 13p. Put your own cylinder and rate through the running cost calculator using the element's wattage and the time it runs.
| Cylinder size | Energy for a full reheat | Standard rate (26p) | Economy 7 night (13p) |
|---|---|---|---|
| 120 L (small or flat) | ~7 kWh | ~£1.80 | ~90p |
| 150 L (typical household) | ~9 kWh | ~£2.35 | ~£1.15 |
| 210 L (large or family) | ~12 kWh | ~£3.15 | ~£1.55 |
Energy to heat the tank from cold mains to a typical storage temperature; in practice you often top up a part-cooled tank rather than heat fully from cold, so a real reheat tends to use less. Economy 7 night rates vary by tariff and region, so check your own. The off-peak column shows the prize: the same hot water for about half the price.
What it adds up to over a year
One tankful is a couple of pounds; relying on the immersion for all your hot water is where it bites. A household heating roughly a tankful a day can spend somewhere around 600 to 850 pounds a year on the standard rate, depending on cylinder size and how much hot water you draw, and roughly half that if the heating is done overnight on Economy 7. As a backup used only when the boiler is down, the cost is trivial. As a main water heater on a flat daytime rate, it is one of the larger lines on the bill, which is the figure worth knowing before you decide how to run it.
The quiet money-waster
The classic and costly mistake is a household with a gas boiler that also has an immersion fitted as backup, left switched on permanently. It silently tops the tank up with expensive electricity whenever the water cools, doing a job the boiler would do for a third of the price, and most people never realise it is happening. If that describes your setup, make sure the immersion switch is off unless the boiler is actually out of action. Flicking that one switch off can shave a surprising amount off the electricity bill, and it is the first thing to check if your electricity use looks higher than it should.
The Economy 7 trick
There is one situation where an immersion genuinely makes sense as a main source, and the table hints at it. If you are on an Economy 7 or similar tariff with cheap night-time electricity, heat a well-insulated tankful overnight at the low rate and the stored hot water lasts much of the day, sidestepping the expensive daytime price entirely. This is how all-electric homes without gas keep their water-heating cost down. The catch is that the cheap night heat has to survive until you use it, so a good cylinder jacket and lagged pipes are essential; without them the tank cools through the day and you end up paying the dear daytime rate for a top-up, throwing away the saving. Cheap heat stored well is the whole game.
Use a timer, and size it to your use
Where the immersion is your main water heater, a timer is the difference between a sensible bill and a wasteful one. Set it to heat a tankful when you need it, once overnight on Economy 7 or once in the morning and perhaps again before evening, rather than maintaining a hot tank around the clock, which just pays to replace heat leaking from the cylinder hour after hour. Size the heating to your real use: a household that showers in the morning needs the water hot then, not at midnight and again at noon. Pair the timer with the cylinder jacket so each heated tankful stays hot between draw-offs instead of cooling and triggering another costly reheat. A timer, a jacket and the right tariff together turn the immersion from a money pit into a workable system.
The bottom line
An immersion heater costs around 1.80 to 3.15 pounds to heat a full tank at the standard rate, depending on cylinder size, and about half that overnight on Economy 7, because the element is efficient but electricity is dear. As a backup it costs next to nothing; as a main heater on a daytime rate it can run to several hundred pounds a year. Switch it off if a gas boiler can do the job, and if it is your main source, heat it overnight on a cheap night rate through a timer, with the cylinder well lagged so the cheap heat lasts. Run that way, even electric water heating need not be a frightening bill.