Worked example

Power1,500 W
Daily use6 hours
30-day energy270 kWh
At $0.17/kWh$45.90
kWh = watts ÷ 1,000 × hours; cost = kWh × rate

Watts measure power; kWh measure energy

Power is the rate at which an appliance uses energy. Utility bills charge for energy over time, usually in kilowatt-hours.

Divide watts by 1,000 to convert to kilowatts, then multiply by hours of operation.

Use the effective rate from your bill

A simple calculator uses one rate per kWh. Real bills may add fixed charges, taxes, tiered rates or time-of-use prices.

For a closer marginal estimate, use the energy charge that applies to the next kWh. For a whole-bill estimate, include applicable non-energy charges separately.

  • Check whether the rate changes by season
  • Use peak and off-peak rates separately
  • Do not confuse cents per kWh with currency units

Cycling appliances need average power

A refrigerator, heat pump or air conditioner does not necessarily draw its rated maximum continuously. Runtime percentage and compressor cycling matter.

A plug-in energy meter or manufacturer usage data can provide a better average than the nameplate maximum.

Use your numbers

Turn the example into your estimate.

The calculator keeps every assumption visible and updates the result as you type.

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