Watts, Amps & Volts Calculator

The power formula: watts, amps and volts

Electrical power in watts equals voltage in volts multiplied by current in amps: W = V × A. Rearranged, amps equal watts divided by volts and volts equal watts divided by amps. That single relationship lets you work out how much current an appliance draws, how much power a circuit can deliver, or what supply voltage a device needs. It is exact for direct-current (DC) circuits such as batteries, solar panels and car electrics.

AC circuits and power factor

In alternating-current (AC) circuits the current and voltage can fall out of step, so only part of the volt-amps is turned into useful work. The fraction is the power factor, a number between 0 and 1. Heaters and incandescent lamps have a power factor close to 1; induction motors and some power supplies are often between 0.6 and 0.9. For single-phase AC the formula becomes W = V × A × PF. For three-phase systems, widely used in industry and for large appliances, it is W = √3 × V × A × PF, where V is the voltage between two lines.

What to use the result for

The calculator also shows kilowatts, apparent power in volt-amps (the figure used to rate transformers, UPS units and generators) and mechanical horsepower. Knowing the current helps you choose a suitable breaker and cable size: a 3 kW kettle on 230 V draws about 13 A, which is why it needs a properly rated circuit. Always size protection devices and wiring with a safety margin, and for long cable runs check the voltage drop as well.

These calculations are for planning and learning; installation work must follow your local electrical code and be done by a qualified person. Enter any two values and the third appears instantly. Everything is computed locally in your browser, so nothing is uploaded.