Generator size calculator

Capacity scenario = (running watts + additional simultaneous surge watts) × (1 + spare fraction). kVA = watts ÷ power factor ÷ 1000.

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Total running load (watts)

Largest additional starting surge (watts)

More inputs and assumptions

Spare capacity (%)

Power factor

Answer

…

Capacity scenario = (running watts + additional simultaneous surge watts) × (1 + spare fraction). kVA = watts ÷ power factor ÷ 1000.

More results

Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

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Formula and assumptions

Enter additional surge above running power, not the appliance’s total starting rating. Add together surges that can occur simultaneously. The spare percentage is your scenario assumption. This does not model motor starting duration, generator transient response, fuel derating or connection safety.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-29

Using Generator size calculator

Capacity scenario = (running watts + additional simultaneous surge watts) × (1 + spare fraction). kVA = watts ÷ power factor ÷ 1000.

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A worked example

For total running load (watts) = 4000, largest additional starting surge (watts) = 2000, spare capacity (%) = 20, power factor = 0.8, power capacity scenario (kw) = 7.2.

Before you use the result

Example results for total running load (watts)

These examples use Largest additional starting surge (watts): 2000; Spare capacity (%): 20; Power factor: 0.8. They are reference calculations, not recommended settings.

Total running load (watts)	Power capacity scenario (kW)
2000	4.8
4000	7.2
8000	12

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