The short answer
Price a water heater from measured electrical kWh or a declared annual electricity figure with its test context. For a resistance element, input kW times actual on-hours can estimate energy. A timer window is not necessarily heating time, and useful hot-water heat is different from purchased electricity.
Use the electric water heaters calculatorOn this page
Choose an electrical input with a clear boundary
The strongest practical input is representative electricity measured for the water-heating system you want to price. Keep the meter boundary, operating mode, measurement dates and hot-water routine. A tank, circulation pump and associated controls may not all sit behind the same meter.
A declared annual electrical figure can also be priced directly, but retain its draw profile and other test conditions. EU water-heater labels distinguish relevant product and load-profile context. Do not enter a fuel figure, useful heat or efficiency percentage as electrical kWh.
Reprice a complete-day observation
For illustrative readings of 42 kWh across seven complete days, average electricity is 6 kWh/day. At a selected $0.20/kWh, that observed sample costs $8.40. Projecting the same daily average onto 30 use days gives 180 kWh and $36.
The sample and projection are different quantities. The numbers are chosen arithmetic inputs, not a Watt & Home household test. Seasonal inlet temperature, occupancy and demand may make another period different. Retain the raw readings in the journal rather than saving only a rounded daily value.
Timer-enabled hours are not element-on hours
A 3 kW resistance element actually drawing that input for two hours uses 6 kWh. A timer allowing operation between two times does not establish those two hours of active heating: a thermostat may stop heating earlier or cycle later.
Use actual on-time or measured kWh. If two elements operate simultaneously, include their combined electrical input for that interval; if they operate sequentially, separate the intervals. Nameplate watts times an entire day can overstate use when heating is intermittent.
Use a volume calculation only as a bounded scenario
The calculator also provides a simplified resistance-heating scenario. It estimates useful water heat from volume and temperature rise, divides by an explicit conversion efficiency, then adds a separately entered electrical loss allowance. It does not predict a tank’s heat loss from volume alone.
For an illustrative 100 litres heated from 15°C to 55°C, assumed density 1 kg/litre and heat capacity 4.186 kJ/(kg·°C), useful heat is approximately 4.651111 kWh. At an assumed 100% resistance conversion and an additional 1 electrical kWh/day of losses, input is 5.651111 kWh/day. Thirty days at $0.20/kWh costs approximately $33.91.
Those temperatures describe the arithmetic task. They are not thermostat or hygiene recommendations. The efficiency input is not a heat-pump COP or a label UEF. See storage losses and energy boundaries.
Heat pumps need a compatible input
Measured or declared electrical consumption can price a heat-pump water heater. A resistance-only volume scenario cannot infer its efficiency, backup-element use or effect on the surrounding space. Use the published field-study example to understand why test context matters.
Hardwired equipment requires appropriate measurement and qualified installation work where relevant; do not connect an incompatible high-current load through a socket meter. The tool estimates cost and does not size equipment, specify a safe temperature or design a circuit.
Understand a changed inlet-water assumption
In the simplified resistance scenario, an additional 20°C temperature rise for 100 litres adds 100 × 20 × 4.186 ÷ 3,600 = about 2.325556 thermal kWh before conversion and additional electrical losses. That is an explicit task change, not evidence that a particular household’s seasonal use rises by that amount.
This example varies a temperature difference only. It is not a recommended storage or outlet setting. For a heat-pump unit, the resistance scenario cannot infer how compressor or backup-element operation responds; use compatible measured or declared electrical consumption instead.
Keep a system-boundary record
Identify whether the meter covers the water heater alone, controls or another component. Measured input already contains electrical consumption within that boundary. A circulation pump outside it needs a separately identified, non-overlapping source if included in a household plan.
The official product context and published study record illustrate why mode, test profile and operating circumstances matter. A declared year, a household sample and a useful-heat scenario should remain distinct when reporting the result.