Heating water is one of the few household calculations with an exact answer. Water has a specific heat capacity of 4.186 kilojoules per kilogram per degree Celsius, and at normal temperatures one litre weighs one kilogram, so the energy needed is litres times 4.186 times the temperature rise, in kilojoules. Dividing by 3,600 converts kilojoules to kilowatt hours, because a kilowatt hour is 3,600 kilojoules.
The temperature rise is what varies. Mains cold water in the UK arrives somewhere between 5 degrees in midwinter and 15 in late summer, and a cylinder is normally stored at 60 degrees to keep legionella at bay. That gives a rise of 45 to 55 degrees depending on the season, which is why a winter bill for hot water is visibly higher than a summer one even with identical usage. Both temperatures are editable inputs here.
Heating 150 litres from 10 to 60 degrees needs 150 × 4.186 × 50 ÷ 3600 = 8.72 kWh. At 24.5 pence per kWh that is £2.14 a day, or £779.86 a year, before accounting for the standing heat loss of the cylinder and the efficiency of whatever is doing the heating.
The formula
litres- Volume of water heated per day
4.186- Specific heat capacity of water in kJ/kg°C
ΔT- Target temperature minus inlet temperature
3600- Kilojoules in one kilowatt hour
efficiency- Efficiency or CoP of the heat source
How to use this calculator
- Enter the litres of hot water you use per day.
- Set the mains inlet temperature and your storage or delivery temperature.
- Choose the heat source, which sets the efficiency or CoP.
- The gauge shows cost per year; daily energy and standing loss follow.
Worked examples
150 litres a day from 10°C to 60°C
The rise is 50°C, so energy is 150 × 4.186 × 50 ÷ 3600 = 8.72 kWh. On an immersion heater at 100 per cent, plus 1.5 kWh of standing loss, you buy 10.22 kWh a day — £2.50 daily and £914.00 a year.
Winter against summer inlet
At a 5°C winter inlet the rise becomes 55°C and the energy 9.59 kWh; at a 15°C summer inlet it is 7.85 kWh. That is a 22 per cent seasonal swing from the same volume of water.
How to read your score
Frequently asked questions
Why 4.186 rather than 4.2?
Because 4.186 kJ/kg°C is the accepted value near room temperature, and it is worth keeping the precision when you multiply by hundreds of litres. It does drift slightly with temperature, but not enough to matter for a domestic estimate.
Can I turn the cylinder below 60 degrees to save money?
The energy saving is real and proportional to the reduced temperature rise, but 60 degrees is the recommended storage temperature because legionella bacteria are killed reliably above about 55. The usual compromise is to store at 60 and fit a thermostatic blending valve to deliver water at a safe 45.
What is standing loss and can I reduce it?
It is heat escaping through the cylinder walls whether you draw water or not. A modern factory foam-insulated cylinder loses roughly 1 to 2 kWh a day; an old copper cylinder with a loose jacket can lose three times that. Adding a proper jacket is one of the cheapest energy savings available.
Is gas cheaper than an immersion heater for this?
Per kilowatt hour, usually much cheaper, though a boiler is only around 90 per cent efficient at heating a cylinder against the immersion heater’s 100. Because the gas unit rate is typically a quarter of the electricity rate, gas still wins comfortably unless you have surplus solar generation.
Does a shower use less than a bath?
A typical eight minute shower at 10 litres a minute is 80 litres, against 80 to 120 for a bath, so a short shower wins but a long one at high flow does not. Change the litres input to compare your own habits directly.
Water heating energy reference
| Volume | 30°C rise | 40°C rise | 50°C rise | £ at 50°C rise |
|---|---|---|---|---|
| 10 L | 0.35 kWh | 0.47 kWh | 0.58 kWh | £0.14 |
| 50 L | 1.74 kWh | 2.33 kWh | 2.91 kWh | £0.71 |
| 80 L | 2.79 kWh | 3.72 kWh | 4.65 kWh | £1.14 |
| 100 L | 3.49 kWh | 4.65 kWh | 5.81 kWh | £1.42 |
| 150 L | 5.23 kWh | 6.98 kWh | 8.72 kWh | £2.14 |
| 200 L | 6.98 kWh | 9.30 kWh | 11.63 kWh | £2.85 |
| 300 L | 10.46 kWh | 13.95 kWh | 17.44 kWh | £4.27 |
One litre of water is taken as one kilogram. Add cylinder standing loss on top.