Is it cheaper to leave an immersion heater on a timer or run it manually?

A timer almost always wins, because it stops the heater running on longer than you need. Manual switches rely on someone remembering to turn it off, which is exactly how it ends up heating water all day by accident.

How much electricity does an immersion heater use in a year?

It depends entirely on how often you use it. As an occasional backup for a few hours a week it might add £20 to £40 a year. Used as a main hot water source daily, it can run into several hundred pounds, since electricity remains pricier per unit than gas.

Does turning down the thermostat on the immersion heater save money?

Yes. Most cylinder thermostats are factory set around 60 to 65°C, which is higher than most households need. Turning it down a few degrees cuts the energy needed to reach temperature, while 60°C is still hot enough to control bacteria in the tank.

Is a 3kW or 6kW immersion heater better value?

Neither is inherently cheaper, since they use the same amount of energy to do the same job, just at different speeds. A 6kW element heats faster but draws more power at once, so the total immersion heater cost for a full tank is roughly the same either way.

Will the October price cap rise make my immersion heater cost more?

Slightly. The cap's 4% rise from 1 October pushes the electricity unit rate up, so running costs will be a little higher than over the summer, even with electricity's VAT-free status continuing until March 2027.

Immersion Heater Cost: 3 Simple Checks to Avoid Overpaying

Guide Contents

Written by

Kim September 17, 2026

Kim covers household energy and utilities for Switcheroo, from the Warm Home Discount to standing charges and the small print behind your bill. Her mission is simple: stop the Do Nothing Default costing you money.

What does an immersion heater actually cost to run?

A standard 3kW immersion heater costs around 79p an hour to run, based on the October 2026 price cap rate of 26.32p per kWh. Run it for a typical two hour heat to fill a hot water cylinder and you’re looking at roughly £1.58. Leave it on all day by accident and that adds up fast.

That number matters more this autumn than usual. The price cap rises again on 1 October, and even though electricity carries no VAT until March 2027, the immersion heater cost isn’t falling. It’s still one of the priciest ways to heat water in a British home.

How the October price rise changes the sum

From 1 October to 31 December 2026, the price cap sets electricity at 26.32p per kWh with a standing charge of 54.83p a day. Gas, by contrast, sits at 7.97p per kWh with a 29.68p daily standing charge.

That gap, electricity at more than three times the price of gas per unit, is the whole story of the immersion heater cost. The VAT removal on electricity softens the blow slightly, but it doesn’t close a gap that size.

Is an immersion heater cheaper than your boiler for hot water?

No, not for most homes, most of the time. Heating three kWh of water through a reasonably efficient gas boiler (around 85% efficient) needs about 3.5 kWh of gas, which comes to roughly 28p at the October rates. The same three kWh through an immersion heater costs around 79p. Gas wins by a wide margin, VAT-free electricity or not.

The exception is when you’re not comparing like for like. If your boiler is broken, if you have no gas supply at all, or if you’re on a tariff like Economy 7 where the immersion runs on cheaper overnight rates, the sum changes. We’ve broken down how those tariffs work in our Economy 7 guide, worth a look if your immersion heater is doing more than occasional backup duty.

Key takeaways

A 3kW immersion heater costs around 79p an hour at the October 2026 price cap rate.
Gas heats the same water for roughly 28p, even with no VAT on electricity.
Leaving an immersion heater switched on permanently is one of the quietest ways to inflate a bill.
A boost timer or a plug timer switch is the cheapest fix, often paying for itself within weeks.
Cheaper overnight tariffs like Economy 7 can flip the sum in the immersion heater’s favour.

The mistake that quietly costs the most: leaving it switched on

Immersion heaters are usually wired with their own dedicated switch, separate from the boiler, and that’s exactly why so many run longer than anyone intended. A previous occupant, a helpful relative, or a switch nobody thought to check can leave it heating water around the clock.

Run a 3kW immersion heater for two hours a day, every day, for a month, and the immersion heater cost comes to roughly £47. Most households only need it to top up hot water after a boiler breakdown, not as a daily routine, so that figure is a warning sign more than a typical bill.

Doing nothing here has a real price. Checking the switch takes ten seconds. Not checking it can cost you a month’s worth of takeaways for hot water you didn’t need.

Should you switch off your boiler and use the immersion heater instead?

For most homes heated by gas, no. The numbers above show gas comes out cheaper even after October’s rise, so switching your hot water over to the immersion heater full time would raise your bill, not lower it.

It’s a different story if you already heat your home on electricity only, if your boiler is out of action, or if you’re testing an occasional boost routine to see if you actually use less hot water than your habits assume. In those cases, a cheap timer switch that limits the immersion to a set window is the simplest way to keep the immersion heater cost under control.

Immersion heater cost at a glance (October 2026 rates)

Heater size	Cost per hour	2 hour heat
3kW (standard)	~79p	~£1.58
6kW (larger tank)	~£1.58	~£3.16

These are estimates based on the capped unit rate from 1 October 2026 and will shift with your own tank size, water temperature and insulation. Ofgem publishes the live rates on its price cap unit rates page, worth bookmarking before you do the sum for your own home.

Frequently asked questions about immersion heater cost

Is it cheaper to leave an immersion heater on a timer or run it manually?
- A timer almost always wins, because it stops the heater running on longer than you need. Manual switches rely on someone remembering to turn it off, which is exactly how it ends up heating water all day by accident.
How much electricity does an immersion heater use in a year?
- It depends entirely on how often you use it. As an occasional backup for a few hours a week it might add £20 to £40 a year. Used as a main hot water source daily, it can run into several hundred pounds, since electricity remains pricier per unit than gas.
Does turning down the thermostat on the immersion heater save money?
- Yes. Most cylinder thermostats are factory set around 60 to 65°C, which is higher than most households need. Turning it down a few degrees cuts the energy needed to reach temperature, while 60°C is still hot enough to control bacteria in the tank.
Is a 3kW or 6kW immersion heater better value?
- Neither is inherently cheaper, since they use the same amount of energy to do the same job, just at different speeds. A 6kW element heats faster but draws more power at once, so the total immersion heater cost for a full tank is roughly the same either way.
Will the October price cap rise make my immersion heater cost more?
- Slightly. The cap’s 4% rise from 1 October pushes the electricity unit rate up, so running costs will be a little higher than over the summer, even with electricity’s VAT-free status continuing until March 2027.

Roo’s take: he’d rather you spent ten seconds checking a switch than ten minutes explaining a bill to your bank. Either way, the fix here isn’t a new supplier, it’s just knowing what your immersion heater cost actually is. For more on where the rest of your bill goes, our appliance running costs guide ranks the worst offenders, and our price cap explainer covers what October’s rise means for the rest of your bill.