Quick answer: kWh = litres × 4.186 × ΔT ÷ 3600. Heating 1 litre by 1 °C takes 0.00116 kWh. A 200 L cylinder from 10 °C to 60 °C needs 11.63 kWh — about 3 h 53 min on a 3 kW immersion heater, or 3.88 kWh of electricity through a heat pump at COP 3.
The formula
m = mass in kg (= litres, since 1 L of water ≈ 1 kg)
c = 4.186 kJ/(kg·K) for liquid water
ΔT = temperature rise in °C (or K — the size of the degree is the same)
kWh = QkJ ÷ 3,600
Worked example — a 200 litre cylinder from 10 °C to 60 °C:
- Mass: 200 L = 200 kg
- Rise: 60 − 10 = 50 °C
- Energy: 200 × 4.186 × 50 = 41,860 kJ
- In kWh: 41,860 ÷ 3,600 = 11.63 kWh
- Time on 3 kW: 11.63 ÷ 3 = 3.88 h = 3 h 53 min
The volume-to-mass step is free because water is the reference substance: one litre weighs one kilogram, one cubic metre weighs one tonne. If you need that in other units, the water weight calculator covers it including the temperature correction.
Why water costs so much to heat
Water's specific heat capacity is 4.186 kJ/(kg·K) — one of the highest of any common substance. It is the reason your hot water is usually the second-largest energy line in a home after space heating.
| Substance | Specific heat kJ/(kg·K) | Relative to water | Energy to raise 1 kg by 50 °C |
|---|---|---|---|
| Water | 4.186 | 1.00× | 209 kJ |
| Ice | 2.09 | 0.50× | 105 kJ |
| Air | 1.01 | 0.24× | 51 kJ |
| Concrete | 0.88 | 0.21× | 44 kJ |
| Steel | 0.49 | 0.12× | 25 kJ |
| Copper | 0.385 | 0.09× | 19 kJ |
| Lead | 0.128 | 0.03× | 6.4 kJ |
Heating a kilogram of water takes eight and a half times the energy of heating a kilogram of steel through the same range. That single property explains why water is used as a coolant in engines and power stations, why coastal climates are mild, and why the immersion heater is the appliance people are told to leave off.
Energy and time for common jobs
Energy required at 100% efficiency, with heat-up time on a 3 kW immersion heater.
| Job | Volume | ΔT | Energy | Time at 3 kW |
|---|---|---|---|---|
| Kettle, one mug | 0.3 L | 85 °C | 0.030 kWh | 36 s |
| Kettle, full | 1.7 L | 85 °C | 0.168 kWh | 3.4 min |
| Washing-up bowl | 8 L | 40 °C | 0.372 kWh | 7.4 min |
| Shower, 8 min | 65 L | 30 °C | 2.27 kWh | 45 min |
| Bath | 100 L | 30 °C | 3.49 kWh | 70 min |
| 120 L cylinder | 120 L | 50 °C | 6.98 kWh | 2 h 20 min |
| 150 L cylinder | 150 L | 50 °C | 8.72 kWh | 2 h 54 min |
| 200 L cylinder | 200 L | 50 °C | 11.63 kWh | 3 h 53 min |
| 250 L cylinder | 250 L | 50 °C | 14.53 kWh | 4 h 51 min |
| 300 L cylinder | 300 L | 50 °C | 17.44 kWh | 5 h 49 min |
| Hot tub fill | 1,500 L | 28 °C | 48.8 kWh | 16 h 16 min |
| Small pool | 20,000 L | 10 °C | 233 kWh | 78 h |
Times assume 100% of the heater's output reaches the water and no heat is lost during the heat-up, so treat them as a floor rather than a prediction. Real cylinder recovery is a little slower.
The pool row is the one worth pausing on. Raising 20,000 litres by 10 °C takes 233 kWh — comparable to a week of total electricity use in many homes, for a single modest temperature rise. It is why pool heating is dominated by heat pumps and covers rather than direct electric heating.
Heat source matters more than the arithmetic
The energy that has to reach the water is fixed by physics. What varies enormously is how much you have to buy to deliver it.
| Source | Efficiency / COP | Typical power | Input for 11.63 kWh of heat | Time |
|---|---|---|---|---|
| Immersion heater | ~100% | 3 kW | 11.63 kWh electricity | 3 h 53 m |
| Electric shower | ~100% | 9 kW | 11.63 kWh electricity | 1 h 18 m |
| Gas boiler, older | 80% | 24 kW | 14.54 kWh gas | 29 min |
| Gas boiler, condensing | 90% | 24 kW | 12.92 kWh gas | 29 min |
| Heat pump, COP 2.5 | 250% | 2 kW | 4.65 kWh electricity | 5 h 49 m |
| Heat pump, COP 3 | 300% | 2 kW | 3.88 kWh electricity | 5 h 49 m |
| Heat pump, COP 4 | 400% | 2 kW | 2.91 kWh electricity | 5 h 49 m |
| Solar thermal | n/a | variable | 0 (fuel is free) | weather-dependent |
Two things this table separates that people routinely conflate.
Efficiency determines how much fuel you buy. Power determines how long you wait. They are independent — and the table is arranged to make that obvious.
Look at the two gas rows: identical 29-minute heat-up, but 14.54 kWh of gas versus 12.92. Both boilers deliver 24 kW of heat, so both fill the cylinder in the same time; the condensing one simply burns less gas to do it. The same pattern appears in the three heat-pump rows — same 5 h 49 min, but 4.65, 3.88 or 2.91 kWh of electricity depending on COP. Efficiency changes the bill, not the wait.
Then compare across: a gas boiler is less efficient than an immersion heater per unit of energy, yet heats the cylinder eight times faster, because 24 kW is eight times 3 kW. And a heat pump is three times more efficient than the immersion heater and the slowest of the lot, at about 2 kW. Efficiency and speed are simply not the same axis.
Power figures here are heat output, which is how boilers and heat pumps are normally rated. If your appliance is quoted by fuel input instead, multiply by the efficiency to get the output before using it.
A 200 litre cylinder delivers 333 litres of usable hot water
This is the part almost every energy calculator leaves out, and it changes how you should think about storage temperature.
Nobody bathes at 60 °C — that scalds. Stored water is blended with cold at the outlet down to about 40 °C. So a cylinder delivers considerably more usable water than it holds:
For 200 L stored at 60 °C, blended to 40 °C with 10 °C mains: 200 × 50 ÷ 30 = 333 litres at the tap. That is about five 65-litre showers from a 200-litre cylinder.
| Stored at | Energy to heat from 10 °C | Delivered at 40 °C | Showers (65 L) |
|---|---|---|---|
| 45 °C | 8.14 kWh | 233 L | 3.6 |
| 50 °C | 9.30 kWh | 267 L | 4.1 |
| 55 °C | 10.47 kWh | 300 L | 4.6 |
| 60 °C | 11.63 kWh | 333 L | 5.1 |
| 65 °C | 12.79 kWh | 367 L | 5.6 |
Notice what dropping storage from 60 °C to 50 °C actually does: it saves 2.33 kWh — 20% of the energy — but cuts usable delivery from 333 to 267 litres, a 20% loss of capacity. The saving is real but it is not free, and if the cylinder then runs out and reheats mid-evening you may use more energy, not less.
There is a safety reason for 60 °C, not just a comfort one. Legionella bacteria multiply in stored water roughly between 20 °C and 45 °C and are killed reliably above 60 °C. Stored hot water is normally kept at or above 60 °C for that reason and blended down at the outlet with a thermostatic mixing valve. Lowering cylinder temperature to save energy is not a decision to make on energy grounds alone — check the guidance that applies to your building, and take advice for any property with vulnerable occupants or long pipe runs.
Common mistakes & pro tips
- Using the target temperature instead of the rise. The formula needs ΔT. Heating to 60 °C from 10 °C is a 50-degree rise, not 60.
- Forgetting the seasonal inlet swing. Mains water might be 5 °C in midwinter and 20 °C in late summer. On a 60 °C target that is a 27% difference in energy for the identical cylinder.
- Confusing efficiency with power. Efficiency sets the fuel bill; power sets the wait. A heat pump wins the first and loses the second.
- Ignoring standing losses. A well-insulated modern cylinder still loses roughly 1–2 kWh a day just sitting there. Over a year that can approach the energy of the water you actually use.
- Comparing gas and electricity by kWh alone. They are usually priced very differently per kWh, so the cheaper fuel and the more efficient appliance are often not the same thing.
- Pro tip — the arithmetic ignores heat loss during heat-up. Real recovery times run longer than calculated, more so for a poorly insulated cylinder in a cold space.
- Pro tip — insulate before you upgrade. A cylinder jacket is the cheapest intervention available and reduces standing loss every single day, whatever heats the water.
How to use this calculator
- Pick the heat source — this sets a default efficiency or COP and a typical power rating.
- Enter the water volume in litres, US gallons or imperial gallons.
- Enter start and target temperatures in °C or °F.
- Adjust power and tariff to match your appliance and energy price.
- Read the blended-delivery row to see how much usable hot water that actually represents.
Frequently asked questions
How much energy does it take to heat water?
Energy in kilojoules equals the mass of water in kilograms multiplied by 4.186 and by the temperature rise in °C. Because one litre weighs one kilogram, heating one litre by one degree takes 4.186 kJ, which is 0.00116 kWh. Heating a 200 litre cylinder from 10 to 60 °C takes 41,860 kJ, or 11.63 kWh.
How long does it take to heat a hot water cylinder?
Divide the energy needed by the heater power. A 200 litre cylinder heated from 10 to 60 °C needs 11.63 kWh, so a 3 kW immersion heater takes about 3 hours 53 minutes at 100% efficiency. A gas boiler delivering 24 kW of heat does the same job in about 29 minutes, because it supplies eight times the power. Efficiency does not change that time — it changes how much gas is burned to deliver those 24 kW.
Why is water so expensive to heat?
Because water has an unusually high specific heat capacity, about 4.186 kJ/(kg·K). That is roughly five times that of steel and four times that of air, so the same temperature rise takes several times more energy than for most other materials. The property that makes water an excellent coolant and a stable thermal store is exactly the property that makes heating it costly.
How much hot water does a 200 litre cylinder actually deliver?
More than 200 litres, because stored water is blended with cold at the tap. A cylinder stored at 60 °C mixed down to a usable 40 °C with 10 °C mains water delivers 200 × 50 ÷ 30, which is about 333 litres at the tap. This is why cylinders are stored hotter than anyone would bathe in, and why storing at a lower temperature reduces usable capacity much faster than it reduces stored energy.
Is a heat pump cheaper than an immersion heater?
Usually yes, per unit of heat. An immersion heater converts electricity to heat at essentially 100%, so one kWh of electricity gives one kWh of heat. A heat pump moves existing heat rather than creating it, with a COP typically between 2.5 and 4, so the same heat costs a third to a quarter as much electricity. The trade-offs are a much lower heat output, so recovery takes longer, and a considerably higher installation cost.
What temperature should a hot water cylinder be stored at?
Stored hot water is generally kept at or above 60 °C to control Legionella bacteria, which survive and multiply in the range between roughly 20 and 45 °C. Water at that temperature will scald, so it is blended down at the outlet, commonly to around 40–43 °C for baths and showers using a thermostatic mixing valve. Reducing storage temperature to save energy is not a decision to take on energy grounds alone.
References & further reading
- Specific heat capacity of liquid water, 4.186 kJ/(kg·K) at typical domestic temperatures — standard thermophysical data.
- Health and safety guidance on the control of Legionella in hot and cold water systems, including stored-water temperature requirements.
- Manufacturer performance data for immersion heaters, condensing boilers and hot water heat pumps.
- Building regulations guidance on thermostatic mixing valves and scald prevention.
Energy figures are exact thermodynamics; efficiencies, COPs and heat-up times are typical values and will differ for your appliance and installation. Nothing here is guidance on Legionella control — follow the rules applying to your building. See our accuracy policy.