Immersion Heater vs Boiler Cost Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

Heating a typical household’s hot water costs roughly to a year by immersion on a standard tariff against to by gas boiler in 2026, which settles the everyday question: with a working gas boiler, the boiler heats water at roughly a quarter of the immersion’s cost. The picture flips in specific circumstances, off-peak overnight rates nearly halve the immersion’s cost, and a solar diverter makes summer immersion water effectively free, which is why the immersion has quietly become the destination for cheap electrons rather than merely a backup.

Annual hot water energy (kWh)

This is the energy your hot water alone needs over a year, separate from space heating: a typical household uses roughly 2500 to 3500 kWh for showers, baths and taps, more with teenagers and power showers, less for one or two careful occupants. If you only want to compare a summer month, a typical household uses 200 to 280 kWh of hot water energy per month.

Immersion electricity tariff

On a standard tariff the immersion pays 24.7 to 26.1 pence per kWh, which is why it has its reputation. On an off-peak tariff, heating a well insulated cylinder in the cheap overnight window nearly halves the effective rate, and smart time-of-use tariffs can do better still on the right nights. Solar households change the maths entirely, since a diverter feeds the immersion surplus generation that would otherwise export for pennies, making immersion hot water effectively free in summer.

Where you live

Heating labour is not priced uniformly across the UK, with published guides putting London and South East installation quotes roughly 15 to 25 percent above the national average. The hardware costs the same everywhere, so the difference sits in engineer day rates, access and parking.

What else affects the figures

The classic myths deserve their corrections: leaving the immersion on all day is not cheaper, since a cylinder simply loses more heat the longer it holds temperature, and heating water twice a day in timed slots beats constant heating for almost everyone; and the boiler-versus-immersion answer can differ in summer, when firing a whole boiler for a small cylinder is at its least efficient, narrowing though rarely closing the gap. The immersion’s genuine virtues are independence, hot water when the boiler fails, simplicity and its appetite for off-peak and solar power, all of which argue for keeping it healthy with a timer or smart control at to fitted. For all-electric homes the immersion versus electric boiler question is really a tariff question, and for gas homes the strategy writes itself: boiler by default, immersion for backup and cheap electricity.

Certification, planning and grid connection

Hot water work splits along a regulatory line: vented cylinders and point-of-use heaters are ordinary plumbing and Part P electrical work, while unvented pressurised cylinders are legally restricted to G3-qualified installers and are notifiable under Building Regulations, because a sealed cylinder of hot water is a pressure vessel whose safety devices and discharge pipework must be right. Immersion heater circuits belong to qualified electricians, water byelaws govern connections and backflow protection, and in hard water areas scale protection is worth specifying at installation. Cylinder thermostats around 60 to 65 degrees Celsius balance legionella safety against scale and running cost.