Generator Size Calculator

Add up what you want to run at once, plus the extra kick for the biggest motor to start, to size a portable or standby generator.

Add the running watts of everything that will run at the same time.
Watts from nameplates.
–
Generator size
Peak starting load
Running load with margin
Current at 240 V / 120 V
Common appliance watts
Work

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How to Size a Generator

Peak load = total running watts − largest motor’s running watts + its starting watts
Generator running rating ≥ running watts × (1 + margin)

Only one large motor usually starts at a time, so only the biggest surge is added. Use nameplate watts; if only amps are given, watts ≈ volts × amps.

Worked Example

Tips

Standby generators connected to house wiring need a transfer switch installed per code. Never run a portable generator indoors or in a garage (carbon monoxide).