Immersion Heater Running Costs UK 2026: Per Hour & Tank

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Immersion heater running costs come down to one sum, same as any other electric element: wattage multiplied by hours multiplied by the price of a kilowatt-hour. At the October 2026 price cap rate of 26.32p per kWh, the standard 3kW immersion element costs 79p an hour to run flat out.

That per-hour figure is not the number most people actually want, though. What you really want to know is what it costs to heat a full tank of water - and that depends on the size of your cylinder, not just the wattage of the element. This guide gives you both: the cost per hour by element size, the cost per tank by cylinder size, and the comparison that decides whether an immersion heater should be your main way of heating water or just a backup.

The rate these figures use

All the costs below use the Ofgem price cap rate for 1 October to 31 December 2026: 26.32p per kWh for electricity, with a 54.83p daily standing charge. There is no VAT on electricity between 1 October 2026 and 31 March 2027.

If you are on a fixed, Economy 7 or time-of-use tariff your rate will differ, so the method matters more than the number: kilowatts x hours x your unit rate in pounds. A 3kW immersion heater for 90 minutes at 26.32p is 3 x 1.5 x 0.2632 = £1.18.

What is an immersion heater, and how much power does it use?

An immersion heater is an electric element that sits inside a hot water cylinder and heats the water directly, the same way a kettle element heats water in a jug. Most UK homes with a hot water cylinder have one fitted as a backup to the boiler, or as the only way of heating water in flats and older electric-only properties.

UK immersion elements are almost always rated at 3kW, sometimes described as the "full" or "long" element because it runs the length of the cylinder. Many cylinders have a second, shorter 1kW "short" or "economy" element near the top, which only heats the top third of the tank - enough for a sink of washing-up or a hand wash without heating the whole cylinder.

The wattage is usually printed on the immersion heater's cap or thermostat housing, or on the paperwork from when it was fitted. If in doubt, 3kW is the safe assumption for a standard UK cylinder.

How much does it cost to heat a full tank of hot water?

This is the figure that actually matters, and it is also the figure almost nothing else online gives you properly - most competitor pages quote a single generic "cost per hour" and stop there, which tells you nothing about your own cylinder.

The energy needed to heat water is fixed by physics: it takes 4.186 kilojoules to raise 1 litre of water by 1°C. Heating a cylinder from a cold mains temperature of around 15°C up to a stored temperature of 60°C - the minimum needed to control Legionella bacteria in a hot water cylinder, as covered in our boiler temperature guide - is a 45°C rise.

So a full reheat of a typical 150-litre cylinder from stone cold costs a little over £2, and takes around two and a half hours on the standard 3kW element. In practice you are rarely heating from completely cold - the thermostat cuts in to top up whatever heat has been lost since the last cycle, which is usually a smaller, cheaper reheat than the full-tank figures above.

Is an immersion heater expensive to run compared to a gas boiler?

Yes, considerably, if you are using it as your main way of heating water rather than a backup. This is the comparison that decides whether an immersion heater makes sense as your everyday hot water source.

Take that same 150-litre cylinder, needing 7.8-7.9 kWh of heat to go from cold to 60°C:

The gas boiler and heat pump figures come from our what temperature should my boiler be set at and heat pump running costs guides, so the comparison is consistent across the site rather than pulled from a different source with different assumptions.

The reason for the gap is the same one that shows up everywhere electricity competes with gas for heat: electricity costs roughly three times as much per unit as gas on the current caps, and an immersion element converts electricity to heat at a 1:1 ratio with no multiplication, unlike a heat pump which moves 3-3.5 units of heat for every unit of electricity it uses. This is exactly the finding in our electric heater running costs guide, applied to water instead of air.

The practical conclusion: if your boiler or heat pump is working, let it heat the cylinder, and keep the immersion heater as a backup for when it is not - a genuinely useful role, not a wasted one. If you have no gas connection and no heat pump, or your only hot water source is electric, the numbers above are simply what hot water costs you, and the savings tactics below matter more.

Using solar power to run your immersion heater for free

If you have solar panels, this is the one scenario where the immersion heater's poor cost-per-kWh stops mattering, because the electricity is free rather than bought at 26.32p.

A solar diverter (sold under names like MyEnergi Eddi or immerSUN) detects surplus solar generation - power your home is not using and would otherwise be exported to the grid at the Smart Export Guarantee rate, typically 4-15p per kWh - and routes it to the immersion heater instead. Heating a 150-litre tank this way, rather than exporting the same 7.8kWh, is worth roughly £2 in avoided electricity spend against maybe 30-60p in lost export income: a clear win whenever the sun is generating more than the house is using. Our solar panel costs guide has the full economics of exporting versus self-consuming, including the battery-storage alternative to a diverter.

How to cut immersion heater running costs

Five things, roughly in order of how much they are worth.

1. Use a timer. An immersion heater with no timer, left switched on permanently on standby-reheat, is the single most expensive way to run one - the cylinder just keeps losing heat to the room and topping itself back up, all day, every day. A 24-hour immersion heater timer switch lets you schedule one or two heating windows a day instead, which is usually the single biggest saving available.
2. Use the boost timer for top-ups, not the main switch. If you only need hot water for one extra shower or a top-up wash, a 2-hour electronic boost timer switches itself off automatically, so you cannot forget and leave a 3kW element running for the rest of the day.
3. Lag the cylinder if it is not already insulated. A cylinder with no jacket or a thin factory jacket loses heat constantly, forcing more frequent reheats. A hot water cylinder jacket costs around £30 and typically pays for itself within a year on an electric-only system, by cutting standing heat loss rather than the cost of any single heat-up.
4. Use the short/economy element for small amounts. If your cylinder has a dual-element setup, the 1kW top element for a sink of washing-up or a hand wash costs a third of what heating the whole tank with the 3kW element does.
5. Check what it is actually costing you. Put a smart plug with energy monitoring on the immersion heater's supply (where it is switched, rather than hardwired) for a week to see the real kilowatt-hours rather than working from the table above. It is the same approach we used to get real figures for electric heater running costs and tumble dryer running costs. Watching your in-home display while the element is on is the fastest way to see the 3kW draw appear and disappear as the thermostat cycles.

Frequently asked questions

How much does it cost to run an immersion heater for 1 hour?

At the October 2026 price cap rate of 26.32p per kWh, a standard 3kW immersion element costs 79.0p an hour at full power. A 1kW "economy" top element costs 26.3p an hour, and a 2kW element costs 52.6p an hour. These are nameplate figures for the time the element is actually drawing power - once the thermostat is satisfied, it switches off rather than running continuously.

How much does it cost to heat a full tank with an immersion heater?

It depends on the size of the cylinder. Heating a 150-litre cylinder from cold mains temperature to 60°C takes around 7.8 kWh, costing roughly £2.07 at 26.32p per kWh. A smaller 100-litre cylinder costs about £1.38 to heat from cold, and a large 210-litre family cylinder costs around £2.89. In everyday use you are usually topping up a partially cooled tank rather than heating from completely cold, so the real per-use cost is often lower.

Are immersion heaters expensive to run?

Yes, relative to gas central heating or a heat pump. Heating the same 150-litre cylinder costs about £2.07 with an immersion heater against roughly 69p with a gas boiler or a heat pump, because electricity costs around three times as much per unit as gas and an immersion element gets no multiplication effect the way a heat pump does. Immersion heaters make most sense as a backup to a working boiler or heat pump, or where the electricity is free - from solar via a diverter, or cheap - on an Economy 7 off-peak rate.

How many kWh does an immersion heater use per hour?

A standard UK immersion element draws 3 kilowatts, meaning 3 kWh for every hour it runs at full power. A "short" or "economy" top element typically draws 1 kilowatt. Multiply the kWh figure by your electricity rate to get the running cost - at 26.32p per kWh, that is 79p for the 3kW element and 26.3p for the 1kW element.

Is it cheaper to leave the immersion heater on all the time or use a timer?

A timer is almost always cheaper. Leaving an immersion heater permanently switched on means the thermostat reheats the cylinder every time it loses a little heat to the room, all day and night, rather than heating it once or twice on a schedule that matches when you actually need hot water. The one exception is a cylinder on an Economy 7 tariff, where a timer set to the cheap overnight window achieves the same result deliberately, at roughly half the standard-rate cost per kWh.

The bottom line

An immersion heater is a simple, reliable way to heat water, and the running cost is straightforward once you know two numbers: the element's wattage and your cylinder's size. Budget 79p an hour for the standard 3kW element, and £2-£3 for a full reheat of a typical UK cylinder from cold.

Where it falls down is cost per unit of heat compared with gas or a heat pump - roughly three times more expensive for the same hot water. That makes it an excellent backup, a good match for solar via a diverter or an Economy 7 off-peak timer, and a poor choice as your everyday primary heating if a working boiler or heat pump is available to do the job instead.

If you want to know exactly what your own immersion heater is costing rather than working from the tables above, our smart meter savings calculator and a week of real readings from a monitoring plug will tell you more than any average ever will.