Every Litre of Hot Water Costs One Kilocalorie per Degree
Hot water is the one household load whose arithmetic you can do in your head, because the kilocalorie was defined from water in the first place: it is the heat that lifts one kilogram of water by one degree Celsius. A litre of water is a kilogram, so the energy a tank, a bath or a tub needs is simply litres multiplied by the temperature rise. The awkward part is that the meter, the tariff and the immersion element are all written in kilowatt-hours, so the tidy kilocalorie answer has to cross over before it means anything on a bill.
Three Things the Water Itself Decides
One Litre, One Degree, One Kilocalorie
The Rise Decides the Cost, Not the Target
The Element Rating Only Sets the Clock
Sizing a Reheat Before You Switch the Immersion On
Four short moves take you from a tank on the landing to a figure you can price against your unit rate.
Multiply litres by the temperature rise
Take the vessel's capacity in litres and the degrees you actually have to climb — final temperature minus the water coming in, not minus room temperature. That product is your kilocalorie figure with nothing else added.
Type the kilocalories and read the meter side
With kcal in one field the kilowatt-hour figure appears beside it keystroke by keystroke, so trying 45 degrees of rise and then 50 costs nothing. A decimal comma from a European appliance manual is read the same as a dot.
Divide the kWh by the element's kilowatts for the wait
A 7.85 kWh reheat on a 3 kW element is 2.62 hours; on a 2.4 kW element it is 3.27. Multiply the kilowatt-hours by your tariff instead and the same figure becomes the price of a full tank.
Swap the arrows when you start from a meter reading
If the smart meter shows 12 kWh went into the hot-water circuit overnight, reversing the direction turns it back into kilocalories — divide by your tank's litres and you have the degrees of rise it paid for.
Household Water-Heating Tasks in Kilocalories and Kilowatt-Hours
Each row is litres multiplied by the temperature rise, converted at 860.42065 kcal per kilowatt-hour, with the last column showing how long a single 3 kW immersion element would need if it were the only heat source.
| Task | Volume | Temperature rise | Energy (kcal) | Energy (kWh) | Time at 3 kW |
|---|---|---|---|---|---|
| Bowl of washing-up | 8 L | 15 → 45 °C (30) | 240 | 0.279 | 6 min |
| Filled bath | 140 L | 15 → 40 °C (25) | 3,500 | 4.07 | 1 h 21 min |
| Small cylinder | 120 L | 15 → 60 °C (45) | 5,400 | 6.28 | 2 h 05 min |
| Standard cylinder | 150 L | 15 → 60 °C (45) | 6,750 | 7.85 | 2 h 37 min |
| Large family cylinder | 210 L | 15 → 60 °C (45) | 9,450 | 10.98 | 3 h 40 min |
| Hot tub from cold | 1,000 L | 15 → 38 °C (23) | 23,000 | 26.73 | 8 h 55 min |
| Small above-ground pool | 12,000 L | 18 → 28 °C (10) | 120,000 | 139.47 | 46 h 30 min |
The volume column carries the whole story. A bath and a cylinder sit within a factor of two of each other, so nobody notices which one they paid for; the tub costs six and a half times a filled bath despite a gentler rise, and the pool is another five times that again. Nothing heats 12,000 litres with a 3 kW element in practice — that last row is there to show why pool owners end up with heat pumps and solar mats rather than a resistance heater.
What Helps While Working Through a Tank
The kWh Column Moves as the Litres Do
Because both boxes recalculate on every keystroke, walking a figure from 5,400 up to 9,450 kcal to compare three cylinder sizes takes no more effort than typing the digits.
A Plain kWh Figure for Your Unit-Rate Sum
The copy control above each box hands over the number alone, with no unit and no thousands spacing, ready for the cell where you multiply by pence or cents. Ctrl+C inside the field does the same.
Turn a Metered Kilowatt-Hour Back Into Litres and Degrees
Reversing the direction converts what the hot-water circuit actually consumed into kilocalories, which divided by tank volume tells you how far the water really rose.
Cylinder Plates That Quote MJ or BTU
Data plates and installation manuals do not agree on a unit. Both dropdowns search the full list of 24 energy units, so a standing-loss figure printed in MJ or a burner rated in BTU still meets your kilocalories on one screen.
Hot-Water Questions From the Airing Cupboard
How many kilowatt-hours does it take to fill a 200 litre cylinder with hot water?
From a 15 °C inlet to a 60 °C store, 200 × 45 gives 9,000 kcal, which is 10.46 kWh. In a cold January, with the mains nearer 7 °C, the rise becomes 53 degrees and the same tank wants 10,600 kcal or 12.32 kWh. Add the tank's own losses while the element runs and 11–13 kWh is a realistic expectation for a full reheat from cold. A tank that has only been partly drawn off costs far less, because you are lifting the replacement litres rather than the whole cylinder.
Why is one kilocalorie per litre per degree the easiest heating rule to remember?
Because the unit was reverse-engineered from the substance. The kilocalorie was defined as the heat that raises one kilogram of water by one Celsius degree, and a litre of water weighs a kilogram closely enough for any domestic sum. That leaves water with a coefficient of exactly 1.000 in these units, so the multiplication disappears entirely: litres times degrees is the answer. Every other material needs its own number, and the joule version of the same rule carries a 4.184 that nobody memorises willingly.
How long does a 3 kW immersion heater need to reheat a full tank?
Divide the converted kilowatt-hours by 3. A 150 litre cylinder climbing 45 degrees needs 7.85 kWh, so 2.62 hours — roughly two and a half hours, or nearer three once losses are counted. A 210 litre tank on the same rise is 10.98 kWh, about 3 hours 40 minutes. This is why cylinders on an off-peak tariff are timed to start several hours before anyone wants a shower, and why the boost switch on a large tank rarely produces hot water as quickly as people expect.
How much does a cylinder lose overnight just standing there?
A modern factory-insulated cylinder of 150–210 litres typically declares a standing loss around 1.5 kWh per day, which converts to about 1,290 kcal — enough to have raised the whole 150 litre tank by roughly 8.6 degrees. Older tanks with a loose jacket can lose two or three times as much. Put another way, a fifth to a quarter of a single full reheat leaks away every day whether anyone runs a tap or not, so the thermostat setting matters most in the tanks that are worst insulated.
Why does a hot tub cost so much more to heat than a bath?
Volume first, then time. A tub holding 1,000 litres is about seven baths, so the first fill from cold is 23,000 kcal or 26.73 kWh even though the target temperature is lower. The bath then drains and costs nothing further; the tub is held at 38 °C for weeks, and every degree it drifts down between sessions has to be bought back. A tub losing just 1 °C a day is spending 1,000 kcal, about 1.16 kWh, purely on standing still — which is why cover condition, not the heater, dominates the running cost.
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