Watts to Volts Calculator

Watts and volts measure different quantities, so power alone is not enough. For DC or a single-phase resistive load, V = P / I. A single-phase reactive load also needs power factor. This calculator does not solve balanced three-phase systems.

It falls straight out of the definition of electrical power. Power equals voltage times current: P = V x I. Solve that for V and you get V = P / I. The units check out the same way: 1 watt = 1 volt x 1 amp, so 1 watt / 1 amp = 1 volt. The formula is exact for DC and for AC purely resistive circuits, where current and voltage are in phase. For AC reactive loads (motors, fluorescent ballasts, transformers), use the real-power form V = P / (I x pf), because not all of the V x I product is converted into useful watts.

Because 120 V is the supply voltage, not the load's power. Watts depend on how much current the device pulls at that voltage, and that varies by device. A phone charger might draw 0.05 A at 120 V (6 W). A microwave might draw 12 A at 120 V (1,440 W). The outlet only sets the voltage; the device decides the current. To find watts from volts alone you need either the current (P = V x I) or the device's electrical resistance (P = V^2 / R). That is why nameplates list both voltage and either amps or watts, never just one.

On an AC reactive circuit (motor, transformer, fluorescent fixture with a magnetic ballast), voltage and current drift out of phase. Apparent power S = V x I, measured in volt-amps, is what the wires must carry. Real power P, measured in watts, is what actually does work. The ratio P / S is the power factor (pf), typically 0.7 to 0.9 for motors. The voltage formula becomes V = P / (I x pf). At pf = 0.8, a 960 W motor pulling 10 A is wired to a 960 / (10 x 0.8) = 120 V supply, not 96 V. If you forgot the pf, you would underestimate the supply voltage and undersize the circuit.

Use the calculation to check a DC supply or a single-phase load when watts, amps, and any required power factor are known. The tool does not implement the balanced three-phase formula, which includes a square-root-of-three factor. Use the equipment nameplate and a qualified electrician for feeder or motor sizing.