How it’s calculated
Electrical power is the rate energy is delivered. Combine P = VI with Ohm's law (V = IR) to get the other two forms. Energy is power multiplied by running time, and cost is kilowatt-hours multiplied by your rate.
Example: a 120 V heater drawing 10 A uses P = 1,200 W. Run 2 h/day for 30 days it uses 1.2 kW × 60 h = 72 kWh, which at $0.17/kWh costs $12.24.
For AC loads with motors or electronics, V × I gives apparent power (VA); real power can be lower because of power factor. The wattage on the nameplate is the better input for those appliances.
Frequently asked questions
How do I convert amps to watts?
Multiply amps by volts: 10 A on a 120 V circuit is 1,200 W. For three-phase circuits use the three-phase formula instead.
What is a kilowatt-hour?
The energy used by a 1,000 W load running for one hour, equal to 3.6 million joules. Utilities bill by the kWh.
Why is my bill different from this estimate?
Bills add fixed charges, taxes and tiered or time-of-use rates, and many appliances cycle on and off rather than drawing full power continuously.
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Sources
- University Physics Volume 2, §9.5 Electrical Energy and Power — OpenStax
- Electricity explained: Use of electricity in homes — U.S. Energy Information Administration
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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