What size generator do you need?

Tick what you want running during a power outage. The calculator adds the running watts, adds the single biggest starting surge on top — the method Generac, EWEB and Champion all print — and shows which portable generators fit and which standby size covers it on your fuel.

How the size is worked out

Every device has a running figure, and motors — refrigerator and freezer compressors, pumps, fans, air conditioners — have an extra starting figure for the few seconds they take to get up to speed. Add every running figure. Then add the one largest starting figure, because the motors are assumed to start one at a time. A portable fits when its running rating covers the first total and its peak rating covers the second. A standby generator has no overload capacity, so it must cover the second total outright, after Generac's derate of about 3.5% per 1,000 ft of elevation and 1% per 10 °F above 60 °F.

The figures are typical values from published tables, not your appliances' own. The nameplate on each appliance (volts × amps = watts) is better when you can read it. The methodology lists every source.

Start here

• Generator size by appliance — refrigerator, sump pump, well pump, furnace, air conditioning, CPAP and more.
• What will a 2,200 to 9,500 watt generator run?
• Standby generators from 10 to 26 kW, with fuel use on propane and natural gas.
• Where to put a generator: at least 20 feet from doors, windows and vents.