Calculate single phase power in watts and kW from voltage, current and power factor, or find the current in amps a single phase load draws.

Use a measured total PF when known; unity is an assumption.

Reactive power and phase
Reactive-power and phase-angle magnitudes require the sinusoidal assumption. Leading or lagging direction is not inferred.

Single Phase Power Formula

The following formula is used to calculate the Single Phase Power.

P = V × I × PF
  • P is the consumed single-phase real power (watts)
  • V is the RMS voltage across the load terminals (volts), whether line-to-neutral or line-to-line
  • I is the RMS current through that load (amps)
  • PF is total real-to-apparent power factor, from 0 to 1 for the nonnegative consuming-load values modeled here

Real power is RMS voltage × RMS current × total PF, and apparent power is S = V × I in VA. Total PF includes waveform distortion and is not generally the cosine of a single phase angle. In Reactive power and phase, selecting sinusoidal voltage and current enables |Q| = √(S² − P²) and |phase| = acos(PF); those magnitudes do not identify leading or lagging direction. Inverse solutions require a nonzero divisor; zero apparent power leaves actual PF undefined.

How to Calculate Single Phase Power?

The following example problems outline how to calculate Single Phase Power.

Example Problem #1

  1. First, determine the input voltage (volts). In this example, the input voltage (volts) is determined to be 16.
  2. Next, determine the current (amps). For this problem, the current (amps) is measured to be 75.
  3. Next, determine the total power factor. In this example, PF is 0.4.
  4. Finally, calculate the Single Phase Power using the formula above: 

P = V × I × PF

Inserting the values from above and solving yields: 

P = 16 × 75 × 0.4 = 480 W

Example Problem #2

Using the same method as above, determine the variables required by the formula. For this example problem, these are:

input voltage (volts) = 98

current (amps) = 73

total power factor = 0.5

Entering these given values into the calculator or the formula above yields: 

P = 98 × 73 × 0.5 = 3,577 W