Immersion Heater Running Costs UK: Cost Per Tank, vs Gas, and Economy 7 Timing

Quick answer: A 3kW immersion heater takes about 6.3kWh to heat a typical 120-litre cylinder from cold — £1.66 at the current cap, or nearer £1.80 once heat losses are counted, in roughly two hours. The same tank of hot water from a gas boiler costs around 59p, which is why timing and tariffs matter so much for immersion users.

All costs on this page are calculated at the October–December 2026 Ofgem price cap for Direct Debit customers: 26.32p per kWh of electricity, with gas comparisons at 7.97p per kWh. Unit rates vary slightly across the 14 GB regions, so treat these as good averages and check the rate on your own bill.

The sum: from litres to pounds

Water needs about 4.2 kilojoules to lift each litre by 1°C. Heating a 120-litre cylinder from 15°C to 60°C — a 45°C rise — therefore takes 120 × 4.186 × 45 ÷ 3,600 ≈ 6.3 kilowatt-hours. At 26.32p per kWh that’s £1.66, and a 3kW element delivers it in a little over two hours. Real cylinders also lose some heat while warming and standing, so budget roughly £1.70–£1.80 per full heat-up from cold.

Cost per tank, by cylinder size

Most households don’t reheat from stone cold every day — topping up a half-warm tank costs proportionally less. But as a rule of thumb, a family relying on a daily 120-litre heat-up pays around £50 a month on standard-rate electricity.

Immersion vs gas: a big gap

The same 6.3kWh of heat from a gas boiler running at about 85% efficiency needs ~7.4kWh of gas, costing roughly 59p — a third of the immersion’s standard-rate price. If you have a working gas boiler, it should normally heat your cylinder, with the immersion kept as a backup or a boost. (Homes with a combi boiler skip this entirely — they heat water on demand and have no cylinder or immersion to worry about.)

Economy 7: the immersion’s best friend

Where immersion heating genuinely competes is on a time-of-use tariff. Economy 7 gives you seven off-peak overnight hours at a much lower unit rate — often somewhere around half the daytime price, though rates vary by supplier and region, so check your own tariff. Heat the full tank overnight on the cheap rate and a well-insulated cylinder keeps it hot for the day; use a timer so the element never runs at peak rate, and keep any daytime “boost” short. Our Economy 7 and Economy 10 guide covers whether the tariff maths works for your household overall.

Cutting the standing losses

• Insulate the cylinder: a decent jacket (or a modern factory-insulated cylinder) sharply reduces the heat that leaks away between uses.
• Set the cylinder thermostat to around 60°C — hot enough for safe storage, not so hot you pay to overheat then blend it back down.
• Fit or use the timer rather than leaving the immersion switched on around the clock; an always-on element quietly reheats every loss at full price.
• Insulate the first metre or two of hot pipework from the cylinder.
• Got solar panels? A solar diverter can dump surplus generation into the immersion for free hot water — see our solar panels guide.

Find your real number in one evening

Your cylinder, thermostat setting and household habits set the true cost, and a smart meter makes it easy to pin down: pick a time when nothing else heavy is running, switch the immersion on for its usual heating window, and read the half-hourly usage afterwards — a 3kW element shows up unmistakably as ~1.5kWh blocks. Do it once from a typical morning state (not stone cold) and you’ll know your actual pence-per-day rather than our table’s worst case. Households with both a boiler and an immersion should also check which one is actually doing the daily work — it’s surprisingly common to find an immersion quietly heating water at 26.32p that the boiler could be making for a third of the price, with the immersion best kept for boiler breakdowns and summer boosts.

FAQs

How much does it cost to leave the immersion on all day?

The element only draws power when the cylinder cools below its thermostat setting, so “all day” really means paying for the tank’s standing losses at 26.32p/kWh — typically an extra 1–2kWh a day (26–53p) for a reasonably insulated cylinder, more for an old bare one. A timer that heats once or twice a day is almost always cheaper.

Is an immersion heater cheaper than gas for hot water?

Not at standard rates — gas delivers the same tank of hot water for roughly a third of the cost. On Economy 7 overnight rates the gap narrows a lot, and with a solar diverter supplying free surplus electricity the immersion can be the cheapest option of all in summer.

How long does a 3kW immersion take to heat a tank?

About 2 hours for a 120-litre cylinder from cold, scaling with size — a 210-litre tank takes closer to 3 hours 40 minutes. Topping up a partly warm tank is proportionally quicker. If you’re unsure what your hot water actually costs, our plain-English bill guide shows how to break it out.