Watts, Amps and Volts Calculator

How it works

In a DC circuit, power is voltage times current: P = V × I. Knowing any two of watts, volts and amps fixes the third.

In a single-phase AC circuit, voltage and current can be out of step. The product V × I is then the apparent power in volt-amperes (VA), and the real power in watts is that times the power factor: P = V × I × PF. A heater, kettle or incandescent lamp has a power factor of 1, so watts equal volt-amperes. Motors and some electronics have a lower power factor, so they draw more current for the same watts.

Pick what to solve for. The calculator reads only the two values that quantity needs, and the power factor only for AC. Three-phase circuits use a different formula and are not covered here.

Formula

DC:               P = V × I
single-phase AC:  P = V × I × PF

I = P ÷ (V × PF)     V = P ÷ (I × PF)     (PF = 1 for DC)
apparent power S = V × I   (VA)

Example

A 1,500 W heater on a 120 V single-phase circuit with a power factor of 1 draws 1,500 ÷ (120 × 1) = 12.5 A, and its apparent power is 120 × 12.5 = 1,500 VA.

A 1,000 W motor load on 230 V with a power factor of 0.85 draws 1,000 ÷ (230 × 0.85) = 5.115 A. Its apparent power is 230 × 5.115089 = 1,176.47 VA, more than the 1,000 W of real power.

Assumptions and limitations

• DC and single-phase AC only. Three-phase power uses √3 × line voltage × line current × PF and needs its own calculator.
• Voltages and currents are steady values: DC levels, or RMS values for AC. Peak or peak-to-peak readings give wrong results.
• The power factor is an input. Typical values (about 0.8–0.9 for induction motors, 1 for resistive loads) are illustrative; a load's actual power factor varies with how heavily it is loaded and is found on its nameplate or by measurement.
• The power factor is treated as a single number. Loads that draw distorted (non-sinusoidal) current, such as many power supplies, have a power factor that includes distortion as well as phase shift; the formula still holds with the true power factor.
• This is an estimate, not a code compliance determination. Conductor sizing, overcurrent protection and installation work are governed by the National Electrical Code (NFPA 70) and local amendments; a licensed electrician or the authority having jurisdiction decides what is acceptable.

Frequently asked questions