What is the rule of thumb for generator sizing?

The rule of thumb for generator sizing: add up the running watts of everything you want powered, add the starting surge of the single largest motor, then put 20 percent headroom on top.

Motor surge is the step people skip. Compressors and pumps demand three to six times their running watts for a second, and the biggest one sets your peak.

Whole-house shortcuts exist, roughly 10 watts per square foot for essentials and double that for everything including cooling, but they are guesses wearing math.

An hour with the nameplate stickers beats every shortcut; generators are priced per kilowatt, and unneeded kilowatts idle expensively for twenty years.

The generator sizing calculator structures that load list and does the surge math correctly.