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Generator Output Calculator

Work out generator output instantly with clear inputs, formula shown and shareable results.

About Generator Output Calculator

A generator set is rated in kVA at a stated power factor, usually 0.8, so real power available is kVA times that factor. Air density falls with altitude, so engine output derates by roughly 1% per 100 m above about 300 m, and the usable kW falls with it.

Formula

Generator output

kW available = kVA x rated pf x (1 - altitude derate); loading = load / available

Worked example

With the default values below, Generator Output Calculator returns Derated real power available of 76.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Generator rating
100 kVA
Rated power factor
0.8
Actual load
62 kW
Site altitude
800 m

Results

Derated real power available
76.00 kW
Loading on the set
81.6% %
Altitude derating applied
5.0% %
Headroom remaining
14.00 kW

Step by step

  1. 1Values used
  2. 2Generator output
  3. 3Derated real power available= 76.00 kW
  4. 4Loading on the set= 81.6 %
  5. 5Altitude derating applied= 5.0 %
  6. 6Headroom remaining= 14.00 kW

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why is the set rated at 0.8 power factor?

Because the alternator is current limited. At a better power factor you cannot exceed the kVA rating, and at a worse one the available kW falls.

Is running a generator lightly loaded a problem?

Yes. Diesel sets below about 30% load suffer wet stacking, glazing and poor efficiency, so sizing too generously causes its own faults.

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