The core energy formula
For a simple appliance, energy in kWh equals power in kW multiplied by operating hours, quantity, and any duty-cycle factor. Cost equals energy multiplied by the applicable electricity rate.
Standby is calculated separately so it is not double-counted during active hours. Calendar periods use 365 days per year and 12 equal normalized months; displayed utility bills may cover a different number of days.
- 1,000 watts equals 1 kilowatt.
- 1 electricity unit equals 1 kWh.
- A 2,000 W heater at full power for 30 minutes uses 1 kWh.
Appliance-specific models
Rated watts are not the best input for every appliance. Refrigerators are strongest when modeled from official annual kWh. Washing machines and dishwashers use kWh per cycle. Water heating uses water volume, temperature rise, specific heat, and efficiency. Cooling and thermostatic heating use duty-cycle or measured-session energy.
The page identifies which model is active and keeps appliance logic outside the interface layer so the same deterministic function can be tested, versioned, and reused.
Tariffs and marginal cost
A flat tariff multiplies energy by one unit rate. A tiered tariff allocates consumption across contiguous slabs. A time-of-use tariff applies the rate for each local period. Fixed charges, taxes, duty, fuel adjustment, subsidies, and minimum bills are separate line items.
For the additional cost of one appliance, Wattwise uses the household’s likely marginal tier when a baseline is known. It does not assign the household’s entire fixed charge to one appliance.
Confidence and ranges
High confidence generally requires measured consumption, an official annual-kWh or per-cycle label, and a known tariff. Typical wattage, assumed usage, weather-dependent duty cycles, or incomplete charges widen the range.
Ranges are not statistical guarantees. They are decision-oriented uncertainty bands derived from source quality and model sensitivity. A result never adds decimal places merely because a computer can produce them.
Versioning and reproducibility
Saved calculations record the engine version, appliance-data version, tariff version, normalized input snapshot, output snapshot, confidence, and time. Historical results can therefore be explained after models or rates change.