The physics covers water heating; cylinder standing loss, pipe loss and reheating add consumption.
The result is not a supplier bill or quote. Unit rates, standing charges, region, payment method, meter type, tariff, VAT, discounts, debt and support schemes can differ. Use values from the same tariff period and bill for a personal check.
Calculate hot-water energy
Do not reduce storage temperatures without considering system hygiene and safety guidance.
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How the calculation works
kWh per heat = litres × temperature rise × 0.001163 ÷ efficiency.
Unit-rate energy and daily standing charges are shown separately wherever possible. That prevents a misleading single pence-per-kWh figure.
Worked example in pounds
Heating 120 litres by 40°C at 90% efficiency takes about 6.20 kWh before standing loss.
The example checks the method. It is not a universal tariff or a forecast of your next bill.
What changes the result
The largest differences usually come from:
- cylinder volume
- inlet temperature
- insulation
- tariff period
- thermostat
How to improve the estimate
Use rates for your region, payment method and meter, including VAT consistently. Match the number of days and do not add standing charges or discounts twice.
Use actual meter readings, separate electricity from gas and compare at least a year for weather-sensitive use. Keep debt repayments and account credit outside consumption cost.
Pre-filled figures are editable, dated reference values with a source.
Frequently asked questions
Will this match my energy bill?
Not necessarily. Regional rates, price changes inside a bill, meter estimates, tariff discounts, debt and rounding can differ.
Does the price cap limit my total bill?
No. It limits rates for covered default tariffs; total cost still rises with energy use.
Sources and review date
Content reviewed on: 2026-08-03.
- GOV.UK – improve home energy efficiency — household efficiency