Electricity cost is a multiplication problem, but each input must represent the same period. Device power becomes energy only after you account for how long the device runs.

01

Convert watts into kilowatts

A watt measures power at a moment in time. Utilities normally bill electricity in kilowatt-hours. Divide watts by 1,000 to convert the device's power rating into kilowatts.

The U.S. Department of Energy describes a kilowatt-hour as 1,000 watt-hours and explains that utility bills charge for the kilowatt-hours consumed.

02

Multiply power by runtime

Energy in kWh equals kilowatts multiplied by hours. A 1,500-watt heater is 1.5 kW. If it draws that power for four hours, it uses 6 kWh.

Thermostats, duty cycles, variable-speed motors, and standby modes make nameplate wattage different from average consumption. A plug-in power meter gives a better measurement for cycling appliances.

  • kWh = watts ÷ 1,000 × hours
  • Period cost = kWh × electricity rate
  • Monthly use = daily kWh × number of days
03

Use the right utility rate

Find the energy rate on a recent bill. A simple calculator generally uses dollars per kWh, but the full bill may also include fixed customer charges, taxes, demand charges, minimums, or tiered and time-of-use pricing.

For comparing two devices, the energy-only rate is often enough. For predicting the exact bill, model the utility tariff and fixed charges separately.

04

Avoid confusing power and energy

A high-wattage device used briefly can consume less energy than a low-wattage device running all day. Compare kWh over the same period, not wattage alone.

The calculator does not estimate carbon emissions or total household consumption unless those values are entered as separate devices.

Bottom line

Convert watts to kilowatts, multiply by actual runtime, and use the energy rate from your own bill.

Frequently asked questions

How do I convert watts to electricity cost?

Divide watts by 1,000, multiply by active hours to get kWh, and multiply kWh by the electricity rate from the applicable billing period.

Does appliance wattage equal actual energy use?

Not always. Thermostats, cycles, variable-speed operation, and standby modes make average consumption different from the maximum nameplate wattage.

Sources and further reading

Sources support the methods and practical guidance above. Product specifications and local requirements should take priority for a specific project.

U.S. Department of Energy: Energy Saver Guide Household energy-use guidance and the role of utility rates and usage. U.S. Department of Energy electricity lesson Watts, watt-hours, and the kWh calculation.