Tick what you'd need to run at once during an outage. Watts are typical estimates — check your own appliance labels for exact figures.
Every appliance has two power figures that matter for generator sizing: running watts, which is the steady power it draws once it's on, and starting (or surge) watts, which is the brief spike a motor needs to get moving — often two to three times its running wattage for a second or two.
Because those surges are brief and appliances rarely all start at exactly the same instant, generator sizing convention only adds the single largest expected surge on top of the total running load, rather than adding every appliance's surge together — which would wildly overstate the generator size actually needed.
A 20% safety margin on top of that combined figure gives the generator some headroom so it isn't running flat-out at its rated maximum continuously, which extends its working life and leaves room for an appliance you didn't think to include.
A generator has to be able to supply that peak surge the instant a motor starts, even briefly, or it will stall or trip. Sizing only for running watts risks a generator that can't actually start your fridge or well pump.
Motors rarely all start at the exact same moment. Standard generator-sizing practice adds only the single biggest expected surge on top of the combined running load, since that's the worst realistic moment the generator has to handle.
Add up the running watts of everything you want to power at once, add the largest single starting surge on top, then add roughly 20% headroom. This calculator does that math automatically as you tick appliances.
They're typical estimates for common household appliances. For an accurate sizing, check the rating label or manual for your specific model, especially for larger loads like a well pump, central AC or furnace.
