Immersion heaters explained

Immersion heaters heat water using electricity. This can be from the mains or renewable electricity you've generated from solar panels.

If you already have an immersion heater, this guide will help you find out how to use it most efficiently and cut costs. If you're considering whether one is right for your home, we outline the pros and cons of immersion heaters, and how you can get free hot water with one.

What is an immersion heater?

An immersion heater is an electric water heater that sits inside a hot water cylinder. It's a bit like a kettle, using an electric resistance heater (which looks like a metal loop or coil) to heat the surrounding water. 

It's separate from your central heating boiler or radiators, and connected to its own power supply via a cable. You can switch an immersion heater on and off easily or connect it to a timer to heat at specific times.

Find out how to buy the best boiler for your household.

Who needs an immersion heater?

An immersion heater can be your primary water heating, for example if your home has electric heating.

Many homes have them as a back-up water heater for a central heating system. This means that, if your boiler fails, you still have a way to heat your water.

How much does an immersion heater cost to run?

Immersion heaters are typically 3kWh. It costs just over £1 to run one for an hour at current electricity prices.

Over a year it could cost around £2,210 to heat enough water for a two to three-bedroom house with a shower or bath, plus £167 in typical daily electricity standing charges.*

That's based on heating a 180-litre tank from 10°C to 60°C twice a day, taking around three hours each time.

How long you'll need to use your immersion heater each day – and therefore how much it costs – depends on:

• the size of your water tank
• how much hot water you use
• when you want hot water to be available
• how much you need to increase the temperature of the water.

Insulating your tank will help keep the water warmer for longer and mean you won't need to increase the temperature as much each time.

* Electricity price used is 34p/kWh (based on the unit rate under the Energy Price Guarantee until 30 June 2023) plus an average standing charge of 46p per day. Costs don't include keeping the tank at the same temperature between cycles and assume 100% efficiency.

How to use your immersion heater efficiently

Leaving the immersion heater on constantly will give you a steady supply of hot water but this is expensive and inefficient.

Follow these tips to ensure you have hot water when you need it and minimise the impact on your bills:

• Insulate your hot water tank so water stays warm for longer.
• Check if you have a thermostatic control, which will automatically turn the heater off when it reaches the desired temperature.
• Set the thermostat at a lower temperature (but ensure that water is heated to a minimum of 50°C to kill any bacteria in the tank).
• Use an immersion heater timer to turn the heater on and off so hot water is ready when you need it. If you have a time-of-use tariff, e.g. Economy 7, set your immersion heater to switch on when electricity is cheaper.

Pros and cons of immersion heaters

Pros

• Immersion heaters are separate from your boiler, so if your boiler breaks down you can still have hot water.
• You'll have hot water for several hours if your tank is well insulated.
• Homes with time-of-use tariffs can time their water heating for when electricity rates are cheaper.
• They're usually easy to switch on and off by flicking a switch.
• Some immersion heaters can use electricity from solar panels so you're heating water for nothing.

Cons

• Heating water using electricity is more expensive than using gas.
• Leaving your immersion heater on all the time is inefficient and pricey, especially if it doesn't have a thermostatic control.

Immersion heater alternatives

Renewable energy

If you have a mainly south-facing roof, solar panels can be another good option for heating your water. Find out more about solar water heating.

Alternatively, if you have an immersion heater and solar panels or a wind turbine generating electricity, you can divert any excess to your immersion heater. 

You'll need to buy a device that does this, such as the Immersun Power Diverter or Solar iBoost+, which cost around £350 to £500 (plus the cost of installation), but you'll be able to heat your water for free.

Boiler

It's generally much cheaper to heat water using gas rather than electricity, so if you have a gas boiler it's likely to be cheaper than using an immersion heater every day. 

If your boiler is quite old and inefficient, consider upgrading to a more efficient combi boiler.

See our reviews of the best boilers to make sure you buy a reliable brand that won't leave you in the cold.