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Calculation methodology
How PlugTally converts watts and kWh into per-use, daily, weekly, monthly and annual UK electricity cost estimates.
Every appliance page uses one shared, tested engine. Watts-and-time mode calculates hours, multiplies watts by quantity and average running percentage, divides by 1,000, then multiplies by duration. Cycle mode multiplies entered kWh per use by quantity. Daily energy is multiplied by 365; annual energy is used directly.
Frequency and periods
Daily uses multiply by 365, weekly by 52, monthly by 12 and yearly by the entered count. The engine produces one annual total, then divides it by 365, 52 and 12. A month is therefore an annual average rather than exactly four weeks.
Tariff and standing charge
Energy in kWh is multiplied by the electricity unit rate in pence per kWh and divided by 100 for pounds. This estimate uses your electricity unit rate. It does not allocate your daily standing charge to individual appliances.
Duty cycle
Average running percentage is available for wattage mode from 1% to 100%. It models appliances whose heater or compressor cycles, but it is an assumption rather than a measurement. Nameplate input can differ from average demand.
Rounding and display
Calculations retain JavaScript numeric precision. Energy displays up to three decimal places. Costs under one penny display in pence; larger costs display in pounds to two decimal places. The displayed rounded periods should not be re-added to reproduce the unrounded annual total.
Defaults and validation
Starting values are labelled examples, not claims. Negative numbers, missing required values, invalid modes and unsafe import shapes are rejected. Decimal comma is normalised only for a simple unambiguous numeric entry. Inputs have generous maximums to avoid accidental overflow.
Limitations
Results are estimates based on the information entered. Actual energy use can vary by appliance model, settings, temperature, cycling, condition and household behaviour. Time-of-use tariffs need separate scenarios or a considered weighted average. Labels use test conditions. The tools do not predict future tariffs, repair costs, whole-bill totals, appliance performance or guaranteed savings.
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