Use a realistic average wattage
A device rated at 500 W may cycle or change power levels. A plug-in power meter can give a more useful average for devices with variable operation.
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Estimate electricity cost from wattage, hours and kWh rate.
Estimate energy use and electricity cost from device wattage, hours per day, number of days and price per kWh. It is useful for comparing the running cost of appliances, computers, heaters and other equipment.
3,720.Many devices do not draw their nameplate wattage continuously. Variable-output appliances such as air conditioners and refrigerators are better estimated from measured average power when available.
A device rated at 500 W may cycle or change power levels. A plug-in power meter can give a more useful average for devices with variable operation.
Use a current bill or provider rate when possible. Time-of-use plans cannot be represented perfectly by one single rate.
The result covers energy use multiplied by the entered per-kWh rate. Fixed monthly charges, taxes, fuel adjustments and other tariff items are separate.
For a computer plus monitor, or several appliances with different schedules, estimate each one with its own wattage and hours, then add the costs.
Keep hours, days and rate constant when comparing an old and new device so the difference comes from energy consumption rather than changed usage assumptions.
Related references: SI prefixes & units · Number formats · Cooking measures