Generator Size Calculator

List every appliance you want to power at the same time and add up their running watts. Then find the single highest starting watts among them and add it to that total. Champion’s guide calls the result the minimum wattage required. In its own example, six appliances add up to 3,880 running watts, the highest starting figure is 2,200, and the minimum is 6,080 watts.

It depends on which appliances you plug in at the same time. As an arithmetic check, the six-appliance example in Champion’s guide needs 6,080 watts, which is below 7,500. Enter your own list in the calculator and compare the total with the wattage rating given by the maker of the generator.

We did not find an open manufacturer or government source that states an 80% rule, so this calculator does not apply one. Champion’s method adds no safety margin either. If you want headroom, ask the maker of your generator and type larger watts into the calculator yourself.

Floor area does not set the electrical load. What matters is which appliances run at the same time. Choose the ones you want during an outage and let the calculator add them. A whole-home standby generator is a different job that this calculator does not cover. Ask its maker or a licensed electrician.

Champion defines running watts as the steady power a device needs to keep working after it has started. Starting watts are the extra power some devices need to turn on. Its guide says starting watts may be estimated at 1 to 2 times the running watts. Virginia Cooperative Extension notes that a motor can draw about three times more current when starting than when running smoothly.

The table is a quick reference, and real appliances vary by model. Virginia Cooperative Extension says the wattage is stamped on the bottom or back of most appliances or on its nameplate. If only amps are listed, multiply amps by volts. Most U.S. appliances use 120 volts, so a 10 amp appliance on 120 volts is 1,200 watts. Larger ones such as clothes dryers and electric cooktops use 240 volts. Type the label number into the calculator.

No. The Consumer Product Safety Commission says never to use a generator inside homes, garages, crawlspaces or sheds, even with fans or open doors and windows, because deadly carbon monoxide can build up in minutes. Run it outside only, at least 20 feet from doors, windows and vents, and use battery-operated carbon monoxide alarms in the home.