Volts to Watts Calculator

Volts and watts measure different quantities, so voltage alone is not enough. For DC or a single-phase resistive load, P = V x I. A single-phase reactive load also needs power factor. Balanced three-phase power uses a different formula and is not modeled by this calculator.

Ohm's law says V = I x R. Combined with P = V x I, it gives P = I^2 x R and P = V^2 / R. This calculator supports the input pairs shown in its mode selector, including V and I, V and R, I and R, P and V, and P and I. It does not solve every possible pair.

Power factor matters whenever the load is reactive, meaning current and voltage are out of phase. Motors (refrigerator compressor, washing-machine drum, HVAC blower), fluorescent and HID lights with magnetic ballasts, and transformers all run at power factors below 1, often 0.7 to 0.9. In those cases the real power in watts is less than V x I, because some energy bounces back to the source each cycle instead of doing work. Real power equals V x I x pf (the power factor). Pure resistive loads (heaters, kettles, incandescent bulbs) run at pf = 1, so V x I gives watts directly.

Apparent power S = V x I is the total volt-amps the wires and transformer must carry. Real power P = V x I x pf is the watts that actually do work. The gap, reactive power Q = sqrt(S^2 - P^2), is what gets shuffled back and forth each AC cycle. Residential customers usually pay only for real power (kWh). Commercial and industrial customers with large motor loads often pay a power-factor penalty when their pf drops below about 0.9, because the utility still has to size wires, transformers, and generators for the full apparent power.

A nominal 120 V, 15 A circuit has an arithmetic maximum of 1,800 VA. Continuous-load rules, other loads on the circuit, receptacle ratings, voltage, wiring, and equipment instructions can reduce what is permitted. Do not use this conversion alone to size or load a branch circuit.