Generator Sizing Calculator

Size a backup or off-grid generator from total running watts, the largest motor's starting surge, and a headroom allowance. Planning estimate only.

Common questions

Do I size a generator to running watts or starting watts?: Both. Running watts is the continuous total; starting watts must cover the largest motor's inrush on top of everything already running. Sizing only to running watts is why generators stall when the well pump kicks on.
How much headroom should a generator have?: Plan on running no more than about 80 percent of rated continuous output. That protects against voltage sag, gives motor surges room, and keeps the unit from running at full load for hours.
Can I back-feed a generator through a dryer outlet?: No. Back-feeding energizes utility lines and household wiring without isolation and is both lethal and illegal in most jurisdictions. Use a transfer switch or interlock installed by a licensed electrician.