Electrical Power Calculator
Calculate watts, volts and amps for any electrical circuit.
Results
The power triangle - watts, volts and amps - is central to every electrical calculation. Whether you are sizing a breaker, checking appliance loads or calculating energy costs, this calculator finds the missing value from any two inputs. It also shows the energy consumption in kilowatt-hours for estimating electricity costs.
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Frequently asked questions
Divide watts by volts: 1500 / 120 = 12.5 amps. This means a 1500W heater needs a 15A or 20A circuit.
Amps = watts / volts. A 2400W appliance at 240V draws 10A. Size the breaker at 125% of continuous load, so use a 15A breaker.
100W for 24 hours = 2.4 kWh. At $0.12 per kWh, that costs about $0.29 per day or $8.64 per month.
At 120V, a 20A circuit can handle 2400W maximum. The NEC 80% rule limits continuous loads to 1920W (16A) on a 20A breaker.
Watts measure real power (work done). Volt-amps (VA) measure apparent power including reactive components. For resistive loads like heaters, W = VA. For motors and electronics, watts are typically 60-90% of VA.