Generator Power Calculator — Real, Apparent & Reactive Power

Formula

How this is calculated

A generator converts mechanical energy (from an engine, turbine or motor) into electrical energy. The key quantity users care about is real power (P, in watts or kilowatts) — the actual work done on the load. For a DC generator the relationship is simply P = V × I. For AC generators a third factor enters: the power factor (PF = cos φ), which captures the phase angle between voltage and current. A purely resistive load has PF = 1 (all power is real); inductive loads such as motors and transformers introduce a lag, reducing PF and raising the apparent power (S = V × I) above the real power.

For three-phase generators — the most common type for industrial and commercial use — the apparent power picks up a √3 factor: S = √3 × V_L × I_L, where V_L is the line-to-line voltage and I_L is the line current. The real power remains P = S × PF. The reactive power Q = √(S² − P²) represents energy that sloshes back and forth without doing net work, and it determines how much current the generator windings must carry above what the load actually consumes.

The efficiency field accounts for losses inside the generator itself (copper losses in the windings, iron losses in the core, mechanical friction). At 90% efficiency the generator must receive 100 W of mechanical input to deliver 90 W of electrical output. This calculator uses the 2025 IEC standard definition of apparent power. Power factor and efficiency values are editable estimates — check your generator nameplate for the rated values.