How much does a hot water cylinder cost to run per year?
Work out what it costs to heat water — a bath, a shower, a sink or a full cylinder — on your own electricity rate, including what an insulated tank loses while it just sits there.
Using the Spanish average of 26.69c/kWh — Eurostat, the second half of 2025.
A 180 litre cylinder including 1.5 kWh a day of standing loss — the heat that escapes into the airing cupboard whether or not anyone runs a tap.
Common cases
What drives the cost
This is the figure that surprises people: a cylinder costs money on days nobody touches a tap. A well-insulated modern tank loses somewhere around 1 to 1.5 kWh a day to the cupboard around it. An older tank with a loose jacket can lose three or four times that, which over a year can approach the cost of the water you actually use.
Two fixes are worth the trouble. A proper 80 mm jacket on an uninsulated tank pays for itself within months, and insulating the first metre or two of pipework leaving the top of the cylinder catches heat that otherwise vanishes into the ceiling void.
How this is worked out
Heating water takes a fixed amount of energy that depends only on how much water there is and how far you are raising its temperature. It takes 4.186 kilojoules to lift one kilogram of water by one degree, a litre weighs about a kilogram, and a kilowatt hour is 3,600 kilojoules — so one litre raised by one degree needs 0.00116 kWh. Multiply by your litres, by the temperature rise, and by your unit rate, and that is the cost. Nothing about this changes from country to country; only the price does.
What this assumes
- Cold water enters at the temperature you set. Mains temperature swings from about 10 °C in winter to nearly 20 °C in summer, so the same bath genuinely costs more in January than in July.
- All the heat ends up in the water. That is true for an immersion heater and close to true for a heat pump cylinder, but a gas boiler loses some up the flue.
- Standing loss is a flat daily figure. In reality it rises with the temperature difference, so a cylinder loses more in a cold house than a warm one.
- Efficiency above 100% means a heat pump, which is not a contradiction: it moves existing heat rather than creating it, so it delivers more heat than the electricity it consumes.
- The rate filled in for you is an average that spreads fixed charges across a year's usage, so it sits above what one extra unit really costs. Entering the unit rate from your own bill gives a closer answer.
Questions
How do I know my cylinder's standing loss?
A modern cylinder has it on the data plate, usually as kWh per 24 hours. If there is no plate, 1.5 kWh a day is a reasonable guess for a factory-insulated tank and 3 to 4 for an older one with a jacket.
Should I leave the cylinder on all the time?
Generally no. A tank held at temperature loses heat every hour, so heating on a timer shortly before you need it costs less. The exception is where leaving it on lets you heat entirely within a cheap overnight window.
Why is 60 °C the usual setting?
Because legionella bacteria survive in stored water between roughly 20 and 45 °C and are killed reliably at 60. That is why cylinders are set there even though nobody uses water that hot — it gets blended with cold at the tap.
Is it cheaper to heat water only when I need it?
For a cylinder, usually yes, because a tank sat at temperature is losing heat continuously. Heating on a timer just before you need it cuts that loss. The exception is if it lets you heat on a cheap overnight rate, which normally beats the standing loss.
Does it cost less to heat water in summer?
Yes, and by more than most people expect. Mains water can arrive nearly 10 °C warmer in summer, which on a 50 degree rise is a fifth less energy for exactly the same tankful.
Does this work for gas water heating?
The energy figure does — the physics is the same. The cost does not, because you would need to enter a gas price per kWh and drop the efficiency to around 80–90% to account for flue losses. This tool is set up for electricity.
Related tools
Sources
- Engineering ToolBox — specific heat of water — checked 2026-08-21
- UK HSE — legionella and hot water temperature control — checked 2026-08-21
Rates from Eurostat for the second half of 2025. Last reviewed 21 August 2026.