POWER · CALCULATOR

Generator sizing calculator

Free generator sizing calculator: get peak load and design capacity from total simultaneous running load, largest additional starting surge and reserve margin.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Total simultaneous running load

Largest additional starting surge

Reserve margin

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

Pdesign = (Prunning + largest extra surge) × (1 + margin)

Assumes one starting load at a time. Check simultaneous starting, altitude, fuel and manufacturer transient limits.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Total simultaneous running load: 3,500 W · Largest additional starting surge: 1,750 W · Reserve margin: 20 %

Peak load: 5,250 W · Design capacity: 6.3 kW

This example uses the stated inputs; results for the entered values appear above.

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Estimating and invoicing tool

Ampxa is developing an estimating and invoicing application for electrical contractors. The list below receives notice when it opens.