How the calculation works
This calculator provides a preliminary load screen only and does not recommend a generator size. Starting demand is estimated as other running loads plus the selected motor running watts multiplied by a user-selected factor. Actual selection requires motor nameplate locked-rotor current/kVA, starting method, alternator transient curves, allowable voltage/frequency dip, load sequence, environmental derating and manufacturer sizing data. Verify the final design with the generator manufacturer and a qualified designer.
Worked example
Example: 6 kW running load and a 1.5 kW motor at 3× gives a rough 9 kW starting-load screen. This does not prove that any particular generator can start the motor.
Common mistakes to avoid
- Adding only running watts and ignoring motor starting surge.
- Sizing to 100% of the load with no margin for future additions.
- Forgetting altitude and temperature derating of engine-driven units.
Frequently asked questions
Starting watts?
Prefer motor nameplate LRA/locked-rotor kVA and manufacturer data. Generic multiples are only rough screening assumptions and cannot select a generator.
Do generators need derating for altitude and heat?
Yes. Engine-driven units commonly lose about 3 to 4% of output per 1,000 ft of altitude and also derate in high ambient temperatures. Check the manufacturer's curves.
Is a generator rated in kW or kVA?
Both. Ratings are usually kW at 0.8 power factor, so kVA = kW ÷ 0.8.
Read the guide
How to Size a Generator for Your Home: Running vs Starting Watts
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For informational and educational purposes only. Always consult a licensed electrician and local code authority. See our Electrical & Legal Disclaimer. Found an error? Tell us.