How much does it cost to fill a sink with hot water?
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 Portuguese average of 24.35c/kWh — Eurostat, the second half of 2025.
A washing-up bowl of about 12 litres at 45 °C. Small each time, but it is one of the most frequently repeated hot water draws in a house.
Common cases
What drives the cost
A sinkful is small enough that the individual figure looks trivial, and frequent enough that the annual one does not. Twelve litres raised thirty-five degrees is a fraction of a kilowatt hour, but done twice a day it is several hundred fills a year.
The comparison worth making is against a dishwasher. A full modern dishwasher typically uses less hot water than the equivalent number of sink fills, which is why it usually wins — the exception being one or two items washed under a running tap, where the sink is far cheaper.
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
Is it cheaper to wash up by hand or use a dishwasher?
A full dishwasher usually wins, because it uses less hot water than most people use at the sink and heating water is the cost either way. Washing a couple of items by hand is cheaper than running a machine for them.
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.