Basic Power Calculator (1Ø / 3Ø, Watts & HP)

Calculate apparent power (VA), true power (W), and theoretical horsepower from voltage, current, phase, power factor, and efficiency.

  1. Set circuit type and voltage Choose single-phase or three-phase, then enter system voltage in volts.
  2. Enter measured current Input load current in amps from meter readings or nameplate validation points.
  3. Add power factor and efficiency Enter power factor (0.01 to 1.0) and efficiency (0.01 to 1.0) to convert apparent power into true output power.
  4. Calculate watts and horsepower Run the calculation to get VA, true watts, and theoretical HP. Use this for planning and cross-checks, not NEC conductor sizing.

How do I calculate three-phase kW from volts and amps?

Use kW = √3 × V × I × PF ÷ 1000. This calculator performs the same relation and also reports apparent power in VA so you can see how power factor affects usable output.

What is the difference between apparent power and true power?

Apparent power (VA) is volts times amps and includes reactive power. True power (W) is VA multiplied by power factor, representing the real work done by the load.

Can I use this calculator to size motor branch-circuit wire?

Not by itself. NEC motor branch-circuit sizing uses table FLA values in Article 430, not only measured volts and amps. Use this tool for quick capacity checks and use the motor circuit calculator for code sizing.

Why does my calculated horsepower not match the motor nameplate?

Nameplate horsepower is rated mechanical output at specified conditions, while this tool estimates theoretical horsepower from measured electrical input and efficiency assumptions. Differences are normal when PF, efficiency, or load conditions vary.

Do I enter line-to-line voltage or line-to-neutral voltage?

For typical three-phase equipment, enter line-to-line voltage (208 V, 480 V, etc.). Single-phase circuits are entered at their nominal circuit voltage such as 120 V or 240 V.

Can I estimate generator loading with this?

Yes as a first pass. Use apparent power and true power outputs to compare against generator kVA and kW limits, then verify starting inrush and transient performance separately.

Calculators