How do I work out what an appliance costs to run?

Watts × hours ÷ 1,000 gives kilowatt-hours; kWh × your rate gives the cost. A 1,500 W heater running 8 hours a day is 12 kWh a day; at $0.15 per kWh that is $1.80 a day, about $54 over 30 days.

What if I only know the amps?

Watts are volts × amps for a heater, kettle or other resistive load, so a 12.5 A heater on 120 V is 1,500 W. For a motor or anything with a power factor below 1, volts × amps overstates the real power — use the watts on the nameplate or a plug-in meter if you have them.

Where do I find my rate?

On your electricity bill, as cents or dollars per kWh. Many bills add delivery and fixed charges; dividing the total bill by the kWh used gives an all-in rate that is closer to what one more kWh really costs you.

Does a thermostat or a cycling compressor change the answer?

Yes. A fridge, a heater on a thermostat or a freezer only draws full power part of the time. Use the hours it actually runs — a plug-in energy meter over a day or two tells you — or put in a duty cycle below 100%.

Electricity cost calculator

Power draw

From the nameplate or a plug-in meter. Or fill volts and amps below and leave this empty.

How many

Volts (optional)

Amps (optional)

Hours a day

Duty cycle

The share of those hours it actually draws full power — under 100 for a thermostat or a compressor.

Your rate

$/kWh

From your bill. 15¢ is 0.15.

Days in the period

— for 30 days

Keep these results

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Type the load in watts — or the volts and amps — how long it runs, and the rate on your bill. The calculator gives the energy in kilowatt-hours and the cost per day, for the period you set, and for a year. Handy for pricing temporary heat on a site, a block heater, a sump pump, or proving to a customer what the old baseboards cost.

Worked example

Two 1,500 W construction heaters run 10 hours a night for a 22-day month at $0.13/kWh. Each is 1.5 kW × 10 h = 15 kWh a night, so the pair is 30 kWh; over 22 nights that is 660 kWh, and 660 × 0.13 = $85.80. With a thermostat holding them at about 60% on, it is 396 kWh and $51.48.

Try one

Quick referencefor energy

Energy: kWh = watts × hours ÷ 1,000
Cost: kWh × rate · a 1 kW load for 1 hour at 15¢ costs 15¢
Watts from amps: volts × amps (resistive loads) · 120 V × 12.5 A = 1,500 W · 240 V × 20 A = 4,800 W
Heat: 1 W = 3.412 BTU/h · a 1,500 W heater ≈ 5,118 BTU/h

The formula

cost = watts × quantity × hours × duty ÷ 1,000 × rate × days

A kilowatt-hour is one thousand watts for one hour, which is what the meter counts and the utility bills. Everything else is multiplying by how many, how long and how much.

Questions

How do I work out what an appliance costs to run?: Watts × hours ÷ 1,000 gives kilowatt-hours; kWh × your rate gives the cost. A 1,500 W heater running 8 hours a day is 12 kWh a day; at $0.15 per kWh that is $1.80 a day, about $54 over 30 days.
What if I only know the amps?: Watts are volts × amps for a heater, kettle or other resistive load, so a 12.5 A heater on 120 V is 1,500 W. For a motor or anything with a power factor below 1, volts × amps overstates the real power — use the watts on the nameplate or a plug-in meter if you have them.
Where do I find my rate?: On your electricity bill, as cents or dollars per kWh. Many bills add delivery and fixed charges; dividing the total bill by the kWh used gives an all-in rate that is closer to what one more kWh really costs you.
Does a thermostat or a cycling compressor change the answer?: Yes. A fridge, a heater on a thermostat or a freezer only draws full power part of the time. Use the hours it actually runs — a plug-in energy meter over a day or two tells you — or put in a duty cycle below 100%.

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