Watts and kilowatt-hours: why electricity cost needs a duration
A higher-powered appliance does not always cost more over a month: operating time matters too. The distinction between watts and kilowatt-hours explains why a power rating alone cannot determine expenditure.
Power at a moment, energy over time
The watt, W, measures power; 1000 watts equal 1 kilowatt, kW. The kilowatt-hour, kWh, measures energy: operating at 1 kilowatt for 1 hour uses 1 kilowatt-hour.
For constant power, energy = watts ÷1000 ×operating hours. Multiply by the number of days for a repeated daily schedule. kW and kWh are not interchangeable on an energy price.
References: US EIA — Measuring electricity
Match the duration to the assumed power
Assuming constant power throughout a period models that scenario; it does not measure actual use. A device that varies its draw or cycles off and on need not use its rated power for every hour it remains plugged in.
Use measured energy where available, or state a consistent power-and-time assumption. Do not combine a high operating rating with every plugged-in hour and call the result measured consumption.
A unit rate is not a whole bill
Arithmetic cost = kWh ×price per kWh. Supply an appropriate rate from its source in one currency. The calculator assumes no live tariff or specific country.
Standing charges, tiers, time-dependent rates or taxes may change the bill. A single supplied price does not fully model a tariff that varies across the period.
Worked example: a constant-power scenario
Assume a device draws a constant 750 watts for 4 hours daily over 30 days. Energy is 0.75 ×4 ×30 =90 kWh. At an illustrative price of 0.30 per kWh, the calculated cost is 27.
If the measured average draw during those hours were half that assumption, energy would be 45 kWh and cost 13.5 at the same rate. This illustrates sensitivity to inputs; it does not claim that any particular appliance actually operates at half power.