Updated · 4 min read
In short
- Energy (kWh) = litres × temperature rise × 0.00116.
- A family of four using 150–200 litres of hot water a day needs roughly 6–8 kWh per day before losses.
- Heat-pump water heaters use roughly 2.5–3.5 times less electricity than an immersion element.
How much energy hot water takes
180 litres a day heated from 10 to 45°C: 180 × 35 × 0.00116 ≈ 7.3 kWh per day, about 2,700 kWh per year, plus cylinder losses. Multiply by your own tariff per kWh to get the cost — for gas, also divide by the boiler efficiency (about 0.9).
The options compared
- Electric cylinder with immersion element: simplest and cheapest to fit, about 100% efficient, but the highest electricity use. Can be timed to off-peak hours or solar production.
- Gas boiler: combi (instant) or with a cylinder. Low energy cost where gas is cheap; combis have a short delay and a limited simultaneous flow.
- Heat-pump water heater: 2.5–3.5 times less electricity than an element; needs space and ambient air of roughly 7–35°C.
- The house heat pump with a large-coil cylinder: efficient, but interrupts space heating while it heats water.
- Solar thermal: covers much of the summer need and less in winter; needs a twin-coil cylinder and a back-up.
- PV surplus: a diverter sends excess solar power to the immersion element instead of the grid.
Cutting use whatever the source
- set the cylinder to 50–55°C with a periodic 60°C+ cycle against legionella;
- insulate hot-water pipes, especially long runs;
- fit low-flow shower heads and aerators;
- check the anode and scale — scale on the element raises consumption.
Frequently asked questions
Is a heat-pump water heater worth it?
It typically uses 2.5–3.5 times less electricity than an immersion element for the same water. It costs more to buy, needs space (ideally an unheated room of a few cubic metres) and makes some noise.
What temperature should the cylinder be set to?
Around 50–55°C for everyday use, with a periodic cycle at 60°C or above to control legionella. Higher settings raise standing losses and scaling.