Generator Size Calculator

Sizing a generator means matching its output to the appliances you actually intend to run, and that figure hinges on two kinds of wattage. Get it wrong in either direction and you either overpay for capacity you'll never use or end up with a unit that stalls the moment a compressor kicks on. The tool weighs both running and starting demands, with particular care for motor-driven equipment.

Calculation starts by totaling the running watts of every appliance you select from the list; each item has a fixed wattage (a refrigerator counts as 700 W, a sump pump as 1,500 W, central air as 3,500 W). Motor loads (the refrigerator, air conditioners, furnace fan, sump pump and well pump) are counted at three times their running watts to start, other loads at their running watts, and the starting figures of everything selected are added together, as if every motor started at once. The recommendation is the smallest of seven standard sizes, from 3,500 to 22,000 W, that covers that total. The fuel line assumes about 0.75 gallons an hour for a 3,500 W unit, scaled by the recommended size and your load (never below a quarter load), over 24 hours at $3.50 a gallon.

Counting only running watts is where most people go wrong. Loads like refrigerators and air conditioners draw far more power to start than to run, so that surge figure matters. Build in a 10-20% buffer for loads you add later or didn't anticipate, and account for the small stuff too, phone chargers and LED fixtures add up once you string enough of them together.