Advanced options
What The Immersion Heater Cost Calculator Works Out
- Energy to heat tank: litres x 4.186 kJ x the temperature rise, divided by 3,600, about 1.16 watt-hours per litre per degree. Lifting a 140 litre cylinder through 45 degrees takes 7.3 kWh. There is no efficiency debate: resistance heating is 100% efficient at the element, and the losses live in the cylinder jacket and pipework instead.
- Cost per full heat: the kilowatt-hours x your unit rate, which defaults to the current Ofgem price cap rate (26.32p per kWh from October 2026): around GBP 2 for the medium cylinder. The same physics is why electric hot water costs two to three times gas-boiler hot water on standard tariffs, and why the immersion's traditional UK role is backup and boost rather than default.
- Heating time: kilowatt-hours divided by the element's 3kW, two hours and a bit for the medium cylinder. Store at 60 degrees, the legionella line, and mix down at the taps rather than storing cooler to save.
- Own cylinder size and element power (advanced): enter the litres from the cylinder label and the element rating; 3kW is standard but some cylinders have two elements or a 2kW top boost.
- On Economy 7 overnight (advanced): the same heat at the off-peak rate, which halves or better the cost. Economy 7 and modern EV-tariff overnight windows turn the well-lagged cylinder into a thermal battery, charged at 3am and spent at 7pm.
- Per year (advanced): the heat cost x full heats per week x 52, at the day rate and all on the off-peak rate. Heat the water you use rather than the clock face.
- Standing heat loss (advanced): what leaks through the jacket every day whether you draw water or not, around 2 kWh a day for a lagged cylinder and two or three times that for a bare one. The lagging jacket is the highest-return purchase in the genre.
- Same water heated by gas (advanced): the kilowatt-hours through a gas boiler at your gas rate and boiler efficiency, with the multiple that electricity costs.
- Half-tank boost and solar diverter (advanced): a half-tank boost on the lower-element switch serves the washing-up better than a full cylinder, and a PV diverter dumps surplus generation into the tank so the summer cylinder heats for nothing. In hard-water country the element furs and weakens; replacement is a routine plumber's hour and Incoloy or titanium elements earn their premium.