Skip to content
ENExplore Calculators

Generator Size Calculator

Size a generator for a connected load including starting surge.

About Generator Size Calculator

Generator sizing is usually governed by motor starting surge rather than running load. Sizing on running load alone leaves the set unable to start its largest motor. Running a generator below about 30 per cent load causes wet stacking in diesels, so oversizing creates its own maintenance problems.

Formula

Generator Size

Generator kVA = max(running kVA × 1.25, starting kVA) ÷ (1 − derating)

starting kVA
Peak demand while the largest motor starts
derating
Reduction for ambient temperature and altitude

Worked example

With the default values below, Generator Size Calculator returns Generator Rating Required of 34.722. Change any input above and every figure here is recalculated in your browser.

Inputs used

Running loadminimum 0 kW
15 kW
Largest motor started on loadminimum 0 kW
5 kW
Motor starting multipleaccepts 1–8 x
3 x
Load power factoraccepts 0.1–1
0.8
Derating for site conditionsaccepts 0–100 %
10 %

Results

Generator Rating Required
34.722 kVA
Running Load in Apparent Power
18.750 kVA
Peak Starting Demand
31.250 kVA
Nearest Standard Generator Size
40 kVA
Running Loading at That Size
46.9%

Step by step

  1. 1Values used
  2. 2Generator Size
  3. 3Generator Rating Required= 34.722 kVA
  4. 4Running Load in Apparent Power= 18.750 kVA
  5. 5Peak Starting Demand= 31.250 kVA
  6. 6Nearest Standard Generator Size= 40 kVA
  7. 7Running Loading at That Size= 46.9

Assumptions and limits

  • The result is derived only from the 5 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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

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

Frequently Asked Questions

How is Generator Size calculated?

Generator kVA = max(running kVA × 1.25, starting kVA) ÷ (1 − derating) Generator sizing is usually governed by motor starting surge rather than running load. Sizing on running load alone leaves the set unable to start its largest motor.

Why does Generator Size matter?

Running a generator below about 30 per cent load causes wet stacking in diesels, so oversizing creates its own maintenance problems.

What values do I need to enter?

This calculator takes 5 inputs: Running load, Largest motor started on load, Motor starting multiple, Load power factor, Derating for site conditions. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Related Calculators

Generator Size Calculator | Calcrivo