Home Backup Generator Size Calculator

Required details

The defaults are reasonable planning assumptions. Actual appliances vary, so check nameplates when possible.

Extra known running load

watts

Add any equipment not listed above if you know the running watts.

Extra known startup/surge load

watts

Add startup load for pumps, compressors, motors, or other equipment not listed.

Advanced settings

Safety margin

%

A 15–25% margin helps avoid running the generator at its limit.

For essential backup, size for the loads you truly need during an outage, not every appliance in the house.
For hardwired home backup, use proper transfer equipment and qualified electrical work.
If you want to run central air, well pump, sump pump, and kitchen loads together, the required generator size can jump quickly.

Helpful tools for nearby decisions.

Estimate generator wattage for RV, camping, cabin, and outdoor power needs.

Estimate how long a portable power station can run common devices.

Estimate battery runtime from watt-hours, load, and efficiency.