Power Calculator

Electrical power measures how fast energy is transferred in a circuit, expressed in watts (W). One watt equals one joule per second. Whether you are sizing a power supply, checking breaker load, or comparing motor ratings, converting between voltage, current, resistance, and power is daily work for electricians, engineers, and hobbyists. This calculator applies the three classic power formulas and extends them for AC circuits with power factor.

The fundamental relationships are:

P = V × I (power equals voltage times current)

P = V² / R (when resistance is known instead of current)

P = I² × R (when voltage is unknown but current and resistance are known)

For AC circuits, multiply by power factor (PF): P = V × I × PF. Resistive loads like incandescent heaters have PF ≈ 1. Inductive motors and fluorescent ballasts often run 0.7–0.9, meaning apparent power (VA) exceeds real power (W). Enter whichever pair of values you know — the tool fills in watts, kilowatts, and mechanical horsepower (1 HP ≈ 745.7 W).

Worked example: A 120 V AC window air conditioner draws 12 A with power factor 0.85. Real power P = 120 × 12 × 0.85 = 1,224 W (1.22 kW). Running eight hours daily consumes 1.224 × 8 = 9.79 kWh. At $0.15/kWh that costs about $1.47 per day. If you only know the heating element is 14 Ω on 240 V DC, P = 240²/14 = 4,114 W — enough to trip a 20 A breaker if sustained.

Always distinguish real power (watts) from apparent power (volt-amperes) on AC systems. Circuit breakers and wire ampacity limit current, not power factor-adjusted watts. Use the electricity bill calculator to translate appliance wattages into monthly cost, and the watts-to-amps converter when panel schedules list one unit but nameplates show the other.