How to use the Generator Power Calculator
A generator's output is described by three related quantities: apparent power (kVA), real power (kW), and reactive power (kVAR). Apparent power is simply voltage times current — it is what the generator's windings and wiring physically have to carry. Real power is the portion that actually does useful work (spins motors, lights bulbs, runs electronics), and it is always less than or equal to the apparent power because of the load's power factor. This calculator takes voltage, current, and power factor and returns all three power values, plus a recommended generator rating that includes a safety margin for starting surges.
Single-phase vs. three-phase power
- Single-phase: apparent power S = V × I (in volt-amps), and real power P = V × I × PF (in watts). This is the standard formula for a typical portable or residential generator with one live/neutral pair.
- Three-phase: apparent power S = √3 × V × I, and real power P = √3 × V × I × PF, where V is the line-to-line voltage and √3 ≈ 1.732 corrects for the 120° phase offset between the three windings. Three-phase generators are common for commercial and industrial standby power because they deliver more power per amp of current.
- Reactive power: Q = S × sin(θ), where θ = cos⁻¹(PF). This is the power that oscillates between the source and inductive loads (motors, transformers) without doing net work — it still has to be supplied by the generator's magnetic circuit even though it is not "used up."
Sizing a generator with a safety margin
- Never size a generator to exactly match the calculated running load. Motors and compressors draw an inrush current — often 3-6 times their running current — for a fraction of a second at startup, and an undersized generator will stall, trip its breaker, or brown out.
- A 20-25% headroom above the calculated real power (kW) is a common rule of thumb for mixed residential or light-commercial loads; loads dominated by large motors may need more.
- Because generators are rated in kVA on the nameplate, convert your target kW back to kVA (kVA = kW ÷ PF) to compare against the manufacturer's rating.
Frequently Asked Questions
What is the difference between kW and kVA on a generator?
kVA (apparent power) is voltage times current: S = V × I. kW (real power) is the power that actually does work: P = V × I × PF. Since power factor (PF) is usually less than 1, a generator's real power in kW is always less than or equal to its apparent power in kVA. Generators are rated in kVA because the manufacturer does not know the power factor of whatever you plug in.
How do I calculate three-phase generator power?
For a three-phase generator, real power is P = √3 × V_line × I × PF, where V_line is the line-to-line voltage and √3 ≈ 1.732 accounts for the 120° phase offset between the three lines. Apparent power is S = √3 × V_line × I (in VA), and P = S × PF.
What power factor should I use if I don't know the load's actual value?
Purely resistive loads (incandescent lighting, resistive heaters) have PF ≈ 1.0. Mixed household or commercial loads with motors, pumps, and electronics typically run 0.8-0.9. If the equipment nameplate lists a power factor, use that value instead of a general assumption.
Why add a safety margin when sizing a generator?
Motors, compressors, and pumps draw an inrush (starting) current several times higher than their running current for a fraction of a second. A generator sized only for steady-state running load can stall or trip when a motor starts. Adding 20-25% headroom above the calculated running kW gives the generator room to handle starting surges and future load growth.