Circuit Power Calculator
Electrical power is the rate at which a circuit converts energy, measured in watts. By Joule's law, power equals voltage times current, and combined with Ohm's law you can also find the circuit resistance and the energy consumed over time. This calculator takes voltage in volts and current in amperes, applies a power factor for AC loads, and returns power in watts, resistance in ohms, energy per hour in watt-hours, and the apparent power in volt-amperes so you can size components and estimate running cost.
Circuit power formula
Apparent power (VA) = voltage * current
Real power (W) = voltage * current * power factor
Resistance (ohms) = voltage / current
Energy per hour (Wh) = real power * 1 hour
Joule's law gives power as voltage times current. Ohm's law gives resistance as voltage divided by current. For resistive loads the power factor is 1 and real power equals apparent power. Energy per hour is simply the wattage held for one hour.
Electrical power context
- One watt equals one joule per second; one kilowatt-hour equals 3.6 million joules.
- US residential circuits are typically 120 volts for general outlets and 240 volts for large appliances.
- The National Electrical Code limits continuous loads to 80 percent of the breaker rating.
- Resistive loads (heaters, incandescent lamps, toasters) have a power factor of 1.
- Motors and switching power supplies have a power factor below 1; enter that value for accurate real power.
Circuit power: frequently asked questions
What is the formula for electrical power?
Electrical power equals voltage times current: P = V * I, where P is in watts, V is in volts, and I is in amperes. This is Joule's law for a DC circuit or the instantaneous power of any circuit. For a 120-volt circuit drawing 5 amps, power is 120 * 5 = 600 watts.
How do I find resistance from voltage and current?
Ohm's law states R = V / I, where R is resistance in ohms. For 120 volts and 5 amps, resistance is 120 / 5 = 24 ohms. This calculator computes resistance automatically alongside power so you can size components correctly.
Does this work for AC circuits?
P = V * I gives the apparent power (volt-amperes) for an AC circuit. Real power in watts is apparent power times the power factor (P = V * I * PF). For purely resistive loads like heaters and incandescent bulbs the power factor is 1, so the result is exact. For motors and electronics, multiply by the load's power factor.
How much energy does the circuit use per hour?
Energy in watt-hours equals power in watts times time in hours. A 600-watt load running for 1 hour uses 600 watt-hours, or 0.6 kilowatt-hours. This calculator shows energy per hour so you can estimate running cost when you know your utility's per-kWh rate.
Why does my breaker trip below the rated amps?
Standard practice in the US National Electrical Code is to load a circuit to no more than 80 percent of the breaker rating for continuous loads. A 20-amp breaker should carry no more than 16 amps continuously. Use this calculator to confirm your load stays within that limit.
Official sources
- U.S. Department of Energy: Estimating Appliance and Home Electronic Energy Use.
- National Institute of Standards and Technology: SI Derived Units (watt, volt, ampere, ohm).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.