The short answer

Useful heat in hot water and purchased electrical energy are different quantities. Measured water-heater electricity already contains losses inside its meter boundary. A resistance-only water-volume scenario needs explicit conversion and extra electrical-loss assumptions; a heat-pump COP or UEF cannot be substituted into it without a compatible model.

Use the electric water heaters calculator
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Write the energy boundary before the formula

Useful water heat describes the thermal energy delivered to a volume of water. Electrical input describes energy supplied to equipment. The two quantities need not be equal, especially for a heat-pump system transferring heat from its surroundings.

A complete electricity measurement may cover compressor or element operation, controls and some standby losses. A thermal reading at a tap covers another boundary. State what each includes before combining them.

Reproduce a simplified water-energy task

Using a liquid-water approximation of 1 kg/litre and 4.186 kJ/(kg·°C), the useful heat to raise 100 litres by 40°C is 100 × 40 × 4.186 ÷ 3,600 = 4.651111… thermal kWh. The density and heat-capacity approximations support an estimate rather than a detailed tank simulation.

The tool distinguishes litres, US gallons and Imperial gallons, and accepts Celsius or Fahrenheit. It converts a temperature difference correctly: a 72°F rise equals a 40°C rise. Temperature entries describe the selected task, not a recommended thermostat setting.

Keep conversion and additional losses separate

In an illustrative resistance-only scenario with 100% conversion into the water and an additional 1 electrical kWh/day loss allowance, purchased electricity is 5.651111… kWh/day. At 95% assumed conversion it becomes 4.651111… ÷ 0.95 + 1 = 5.895906… kWh/day.

The loss allowance must have a documented or clearly assumed basis. The calculator does not infer tank losses from tank size. Entering zero intentionally creates a loss-free extra-loss scenario; leaving a required loss field blank is not treated as zero.

If an overall measured efficiency already includes the same storage loss, adding the allowance again double counts. The simplified conversion field is deliberately limited to resistance conversion; it does not accept a heat-pump performance factor.

Do not add losses to measured electricity twice

A submeter reading already contains electricity consumed inside its boundary. Price its kWh directly. Do not add a second standby figure because a product document also reports one, unless you establish that it lies outside the observation.

Likewise, a declared annual electrical figure belongs to its tested system, profile and mode. EU water-heater information distinguishes product and load-profile context. The tool prices that annual figure without inventing an adjustment for household timer hours.

A heat pump needs its own compatible evidence

COP, test efficiency and daily energy factors can use different boundaries and conditions. A single maximum value does not describe a whole year of compressor, backup element, storage and control operation. Prefer representative electrical measurements or a suitable declared electrical consumption input for a cost estimate.

The published California field-study guide demonstrates how measured aggregate electrical consumption can vary with season and sample context. It is an external research example, not a prediction for your house.

Turn the estimate into a repeatable record

Retain dates, meter boundary, household routine, equipment mode and temperatures where recorded. The measurement journal can hold genuine observations; it should not be populated with published averages as if they were your meter readings.

The calculator cannot size a tank, design an installation, specify safe water temperatures or infer every system component. For a bill-period check, include only electricity on a consistent boundary and period, with overlapping rows removed.