Immersion heaters – what are they and how do they work?

What is an immersion heater, and is it still worth it in 2025?

Whether you rely on a BUS-funded heat pump, a gas combi boiler, or a simple electric element, the goal is the same: reliable hot water at the lowest running cost. This updated guide explains how immersion heaters work, today’s 2025 electricity prices, sizing maths, the pros and cons of modern boilers and heat pumps, and safety tips such as legionella-safe setpoints.

What are immersion heaters and what do they do?

If you’ve never seen an immersion heater before, you’re probably going to be asking yourself, ‘what is an immersion heater and what’s its purpose?’

In simple terms, immersion heaters are devices that provide hot water for your home, powered by electricity. However, an immersion heater could be your knight in shining armour if your central heating fails, as it can still provide your home with hot water.

Hot water immersion heaters can also be used as your main source of hot water or as a back-up for your traditional gas boiler, as it is separate from your central heating boiler and radiators.

Those who still have older gas boilers or oil boilers in their property often choose to install an electric immersion heater as a safety blanket, so that they always have a supply of hot water.

You will also find that immersion heater tanks are the primary source of hot water in newly built homes that are not supplied by the main gas network or homes that are off the grid as a result of their remote locations.

Immersion heaters are located within a large hot water cylinder, also known as an electric resistance heater that heats the surrounding water. They are connected to the main power supply and can be switched on and off on-demand, meaning you do not have to have it switched on and heating the water constantly.

How do immersion heaters work?

When turned on, electricity passes through the metallic element, heating the surrounding water. This is where they get their nickname ‘the giant kettle’. The immersion heater sits inside a hot water cylinder and is powered by a strong electric current which works to heat the water until it reaches optimum temperature.

It is important to note that to kill any potential pathogen (any nasty bacteria inside the tank) the immersion heater needs to heat the water to at least 50 °C so that the water is safe to use.

Your immersion heater will automatically turn back on and get back to work if the temperature of the water reaches below a certain level, however, for the times that you’re not at home, or during peak electricity hours where the water does not have to be kept warm, you can turn this feature off to save you some extra coin.

The heating process can take anywhere from 1.5 to 3 hours, depending on the size of the cylinder being heated and the power of the immersion heater.

The storage cylinder is filled with cold water which enters through the bottom of the tank. The cold water then rises to the top as it is heated. As more cold water enters the bottom of the tank and the pressure in the tank builds up, the required pressure is reached for the hot water to leave the immersion water heater and find its way to your baths, showers, and sinks.

In addition to using electricity from the mains supply to activate the immersion heater, it is also possible to connect them to solar panels and other renewable energy sources, which can help to reduce the energy costs.

When and how to use immersion heaters

If you feel that your home requires a constant and steady supply of hot water, you may choose to keep your immersion heater on at all times. However, this can be quite costly and inefficient as it uses a lot of electricity to keep the water hot.

What is the most effective way of using an immersion heater? Well, there are a few ways of doing this to ensure that those energy bills don’t start creeping up.

You could set your immersion heater thermostat to a lower temperature so not so much electricity has to be used for the water to reach optimum temperature. However, it is important to remember that it needs to reach a minimum temperature of 50 °C to remove any bacteria.

Setting the thermostat will mean that the immersion heater will stop working once it reaches the right temperature and only power up when it drops below a certain level of heat.

Another way of doing this is by using an immersion heater timer to only allow your immersion heater to turn on during off-peak hours.

The only issue with this is that you will need to pre-plan when you’re going to need hot water for your home, otherwise, you may have to wait a while for the tank to heat up before being able to take a shower, have a bath or even do the washing up.

This leads us on to the next section of the blog, the pros and cons of using immersion heating.

Pros and Cons of using an immersion heater

Cons

• Pre-planning when to turn your immersion heater on. With immersion heaters, unfortunately, you can’t simply heat a small amount of water for perhaps, let’s say, one load of washing up. You will need to wait for a full tank of water to be heated. Not only is this inefficient because once the immersion heater is off, the water will lose its heat and you would have wasted electricity for nothing, but you could also be waiting up to three hours to use the hot water.
• For this reason, combi boilers are often the preferred choice of boilers as they are much more efficient and convenient for homes that have a demand for instant hot water and central heating.
• If you’re interested in learning more about combi boilers, we have another blog to cover this. Read our detailed guide to how combi boilers work.
• They are not always cost-effective. Unless you have an immersion heater thermostat installed, you will have less control over the temperature your immersion heater needs to reach. To avoid the issue of wasting electricity, many immersion heater owners leave them switched on. However, this is one of the most expensive ways of heating water.
• Heat-pump grants now undercut immersion-only running costs. Because BUS reduces a heat pump’s up-front price and modern units deliver 2.5–4 kWh of heat per 1 kWh of electricity, the effective cost of heat can fall to roughly 10–15 p/kWh. That’s about half the 24.9–27.0 p/kWh you pay when an immersion element does 100 % of the work, meaning an immersion-only setup looks increasingly expensive unless you have very low night-rate electricity or excess solar to dump.

Pros

• They’ve got your back. Immersion heaters do not require a gas supply to function, which means that if for any reason your central heating boiler fails, you can rely on your immersion heater back to help supply your home with hot water. Great for that extra peace of mind when it gets absolutely baltic outside!
• No gas supply? No drama. Immersion heaters are a great hot water supply for homes that are off-grid and have no gas supply, for example, flats or homes in remote locations.
• Free hot water with solar panels – and 2025 grants. Solar PV (or solar-thermal) can slash the running cost of an immersion heater because any excess generation can be diverted to the cylinder instead of being exported to the grid. A simple PV-diverter gives you almost-free hot water while you still earn Smart Export Guarantee payments on your export surplus.
• The old Domestic Renewable Heat Incentive (RHI) closed to new applicants on 31 March 2022. Its successor, the Boiler Upgrade Scheme (BUS), now offers up-front grants of up to £7,500 for air- or ground-source heat pumps (and £5,000 for eligible biomass boilers) in England and Wales. Find a Grant. Although the BUS does not subsidise an immersion heater directly, a heat pump can feed the same hot-water cylinder, letting the immersion element act as a top-up on dull winter days while the grant covers most of the heat-pump cost. That combination can be considerably cheaper to run than an immersion-only set-up.
• Easy to manage your energy usage. You have full control of your immersion heater, meaning that you can turn it on and off when you please so that you can limit the amount of energy being used. If you have an immersion heater thermostat installed, it will automatically turn itself off when the water reaches the desired temperature. Once the water cools below a certain temperature, it will then turn itself back on and get back to work.
• You can also install an immersion heater timer, which you can set to automatically turn on at certain times of the day which could save you money, for example, if your energy tariff is cheaper throughout the night, this would be the most cost-effective time for it to be on.

How much do immersion heaters cost?

The cost will depend on the size and the type of immersion heater you choose, although they are relatively inexpensive especially in comparison to other types of heaters. However, it is important to note that you will need to purchase and accommodate extra room in your home for an additional hot water storage cylinder. In addition to this, you will need to hire a Gas Safe Engineer to carry out the installation.

Two types of immersion heaters are more commonly used. These are the:

• Copper immersion heater: These usually range between £20 and £30. It is important to note that this immersion heater is not appropriate for hard water, stainless steel tanks, unvented cylinders, or thermal store units.
• Titanium immersion heater: These usually range between £30–£40. These are used mainly for areas that suffer from hard water.

How much does it cost to run an immersion heater in 2025?

It is difficult to say exactly how much it is going to cost to run an immersion heater, as they use electricity to heat water, which is considerably more expensive than gas. On top of this, you’re adding the cost of heating the whole hot water cylinder and often not using all the water, resulting in wasted energy and wasted money.

Under Ofgem’s latest energy-price cap (1 April – 30 June 2025), the average electricity unit rate for customers paying by Direct Debit is 24.86 p – 27.03 p per kWh. A 3 kW immersion element therefore costs:

• 74 – 81 p per hour on a standard single-rate tariff (3 kW × 24.86–27.03 p).
• ≈ 45 p per hour on an Economy 7 night-rate, where the nationwide April-2025 average is 14.92 p per kWh.

Keeping that same 3 kW element on for two hours daily would add roughly £566 a year (standard tariff) or £327 a year (Economy 7 night-rate) to your bill.

*Assumes the cylinder is heated for two hours every day for a year.

Money-saving tip: Use a timer so the immersion runs only during Economy 7 hours, or connect it to a solar-PV diverter for virtually free hot water. Rates vary slightly by region; check your own supplier’s unit prices.

How do I work out what size immersion heater I need?

Now, I bet you didn’t know that there was more than one option for size when it comes to an immersion heater, well there is and it’s important that you get it right. The size of the immersion heater can also have an impact on the price. It is also important to know that buying an incorrect size heater will not serve its purpose and your water may not ever reach the desired temperature.

Don’t panic, it’s not too hard to figure out what size you need. Start by finding out how many litres of water the cylinder stores and your required temperature increase. Once you have that information, you can determine how much power is required to heat your water in 1 hour using the following equation:

Volume of tank x 4 x temperature increase / 3412 = Power (kW) required

For example:

If you have a 100-litre storage cylinder and want to increase the temperature of the water from 10 °C to 60 °C, the calculation would be 100 x 4 x 50 / 3412 = 5.8 kW

Therefore, to get your water heated to the desired temperature you would require a 5.8 kW immersion heater.

Alternatives to an immersion heater

We know that’s a lot to take in, so we’ve decided to scale it back a bit. If you’ve got to this stage in the blog and figure that an immersion heater may not be the right choice for you, then we’ve come up with an alternative.

Combi boilers are ideal for homes with a high demand for hot water, and they only heat the water that you use, meaning less energy is wasted. Gas is also much cheaper means of heating water than electricity, making a combi boiler much more cost-effective.

Combi boilers are supplied with cold water directly from the mains. They have the central heating and water heater built into one unit meaning you don’t have to wait for the tank to heat up and it saves you space in your home. In addition to this, combi boilers use fresh water, meaning there are no risks of nasty bacteria.

Like what you read? We have an article on the cheapest combi boilers on the market that might help you find the perfect combi boiler for you.

Is an immersion heater right for me?

Immersion heaters have a lot of beneficial features such as supplying hot water in times of need, compatibility with solar panels to help you save money on your energy bills, ideal for homes with no gas supply. However, ultimately, they are expensive to run and you could get the same, if not better results with a combi or system boiler.