Electrical Power, Current and Voltage Calculator (Single and Three Phase)

How is electrical power calculated?

In AC circuits the active power drawn by a load is the product of voltage, current and power factor.

• Single phase: P = V × I × cos φ
• Three phase: P = √3 × V × I × cos φ (V is line to line voltage, I is line current)

This calculator takes three of the four quantities and finds the fourth. It also gives apparent power, reactive power and the phase angle.

Active, apparent and reactive power

• Active power (P, kW): the power turned into work. This is what the energy meter records.
• Apparent power (S, kVA): voltage multiplied by current. Transformers, generators and cables are loaded by this value.
• Reactive power (Q, kvar): power exchanged between the supply and coils or capacitors. It does no work but raises the current.

The three form a right triangle: S² = P² + Q². The power factor is the ratio cos φ = P / S.

Example

A motor drawing 20 A from a 400 V three phase supply at a power factor of 0.85 has an apparent power of 1.732 × 400 × 20 = 13.86 kVA, an active power of 13.86 × 0.85 = 11.78 kW and a reactive power of 7.30 kvar.

Limits

The calculation assumes a balanced three phase load and sinusoidal current. For unbalanced loads calculate each phase separately. A motor nameplate shows shaft power, so the power drawn from the supply also depends on efficiency. Use the kW, kVA, HP and amps converter for that.