Sizing a backup generator means adding total running watts, the largest motor’s starting surge, plus a safety margin.
For the full breakdown, see our best Backup Generator For Sump Pump guide.
A backup generator fails in two ways, and both announce themselves at the worst time: the motor stalls when the well pump kicks in, or the unit idles along burning fuel you never needed. Both failures come from the same mistake — guessing instead of sizing. Here’s how to size a backup generator the way an engineer would: inventory every load, convert amps to watts, add the largest starting surge, and apply a safety margin. The same four steps work for a portable unit that powers three circuits and for a permanently installed whole-home system.
What Does a Load Inventory Look Like?
A load inventory is a room-by-room list of every appliance you’ll run during an outage, with each item’s running watts and starting watts written down. The generator only needs to cover what you actually plan to switch on — nothing more.
Walk the house with a notepad. Most appliances carry a nameplate that lists watts or amps; if you get amps, convert: watts = amps × volts. Never add amps when sizing a generator — a 15-amp, 120-volt window AC draws 1,800 watts and a 12-amp, 120-volt furnace fan draws 1,440, so combine the watts, never the amp figures. For 240-volt appliances, keep the full voltage in the conversion: a 10-