What Size Generator Do I Need?

To size a generator, add up the running watts of everything you want on at the same time, then add the biggest single start-up surge, because motors in fridges, pumps, and furnace blowers draw two to three times their running watts for a few seconds as they start. Tick what you'd run in an outage, and this free calculator does that math, adds a safety margin, and tells you which size of generator fits.

How the Calculator Works

It follows the method generator makers publish. Champion, Generac, Briggs & Stratton, Westinghouse, and FIRMAN all say to add the running watts of everything you'll run at once, then add the single biggest extra starting surge, on the assumption that motors start one at a time.

• Running watts are what an appliance draws while it runs. The generator's running (or rated) watts have to cover the total for as long as it runs.
• Starting watts are the brief surge a motor needs to get going. Briggs & Stratton puts it at two to three times running watts for two or three seconds. The generator's starting (or peak) watts only have to cover the total running watts plus the one biggest surge.
• The figures are the median of the wattage charts published by Champion, Generac, Honda, Westinghouse, Briggs & Stratton, FIRMAN, and DuroMax (the higher of the two middle figures when there's an even number), rounded up, after converting each chart to the same convention: some list the total watts while starting, others only the extra surge. The CPAP figure also uses ResMed's, and the router and modem figures EcoFlow's. The charts disagree a lot: a refrigerator runs anywhere from 75 to 1,000 watts and starts at 1,200 to 3,000, depending on the chart, so your own appliance may need less, or more.
• The margin: Briggs & Stratton advises adding 20% to 30% to the total, and Champion says to leave “a safe margin”. The calculator adds 20% to your running watts, rounded up to the next 50, so the generator isn't run flat out.
• The sizes are the generators in our generator guide, by their gasoline ratings:

If anything on your list runs on 240 volts, the calculator skips the two 120-volt sizes. Generators also make less power on propane and in thin air: Champion says about 10% less on propane, and Generac and Westinghouse say maximum power drops about 3.5% for every 1,000 feet above sea level.

What Size Generator for Common Outage Lists

The same math applied to lists people commonly make, using the figures in the calculator. Each list includes 10 LED bulbs, two phone chargers, and a Wi-Fi router.

That lines up with what the makers say about whole homes. Champion says most homes need a 5,000- to 12,500-watt generator for the essentials, and 10,000 watts or more to run central air. Honda says a house on city water with a gas, oil, or propane furnace and water heater can get by on 3,000 to 4,000 watts, while a house with a well and an electric water heater needs 5,000 to 7,000 watts with 120/240-volt output.

Notice how much the start-up surge drives the answer. Two motors starting at the same moment can overload a generator that runs them both easily, so switch on the biggest motor first and add the rest one at a time.

Connecting the Generator Safely

• Outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors, windows, and vents, as the CPSC says. Never in a garage, basement, crawl space, or shed, even with the doors or windows open, and never on a porch: carbon monoxide builds up fast and lingers.
• Carbon monoxide alarms: battery-powered or with battery backup, on every level and outside sleeping areas. A generator's CO shutoff doesn't replace them. See our smoke and CO alarm guide.
• Never plug a generator into a wall outlet. That's called backfeeding, and the CPSC warns it can electrocute utility workers and neighbors on the same transformer. Plug appliances into the generator with heavy-duty, outdoor-rated extension cords whose rating exceeds the load on them.
• Hardwired loads such as a furnace, well pump, central air, or electric water heater can run only through a transfer switch that a licensed electrician installs. OSHA says never to connect a generator to a building's wiring any other way.
• Space heaters must plug straight into a wall outlet, never an extension cord or power strip, so on a generator they belong only on a circuit fed through a transfer switch. Keep 3 feet of clearance, and turn them off when you leave the room or go to sleep.
• Refuel only when it's off and cool, since gas spilled on a hot engine can ignite, and keep the generator dry, under an open canopy if it's wet out.

For everything else to do when the lights go out, from food safety to staying warm, see our power outage guide.

Generator or Battery Power Station?

A fuel-burning generator is never safe to run indoors. A battery power station has no exhaust, so it can run inside, and it plugs in like a big battery. Its limits are output and capacity: the stations in our power station guide put out 600 to 2,400 watts, and once the battery is empty it needs recharging. When your list fits a station's output, the calculator says so. To estimate how long a battery lasts, Anker, EcoFlow, and Jackery all use the battery's watt-hours times 0.85 divided by the load in watts; use your appliances' average draw, since a fridge cycles on and off and averages far less than its running watts.

Frequently Asked Questions

What size generator do I need to run a refrigerator?

By the median of the generator makers' charts, a refrigerator runs at about 700 watts and needs about 2,850 watts to start, so a fridge plus lights and chargers calls for a mid-size generator of about 3,500 running and 4,500 starting watts. Fridges vary more than anything else on the list: Honda lists one at 1,200 starting watts and says a small generator like its EU2200i can run basics such as a refrigerator, TV, microwave, and lights. Check your fridge's label or manual; a newer, efficient model usually needs less.

What size generator do I need to run a house?

It depends on what has to run at once. Champion says most homes need 5,000 to 12,500 watts for the essentials, and Honda says 3,000 to 4,000 watts for a house on city water with gas heat and hot water, or 5,000 to 7,000 watts with 120/240-volt output for a house with a well and an electric water heater. Running the whole house at once, central air and electric appliances included, takes a bigger generator than the largest portable in our guide, such as a permanently installed standby generator sized by a licensed electrician. Tick your own list above to see where you land.

Can a portable generator run my furnace?

The furnace's blower is the load: about 700 to 800 running watts for a 1/3 or 1/2 hp motor, with a start-up surge as high as about 3,150 watts. But a furnace is hardwired, so it can run on a generator only through a transfer switch installed by a licensed electrician. Never power it any other way.

What size generator do I need for central air?

Champion lists a 3-ton central air conditioner at 3,000 to 3,500 running watts and 5,000 to 6,000 extra watts to start, and says to plan for 10,000 watts or more. It's hardwired and usually runs on 240 volts, so it needs a transfer switch installed by a licensed electrician. Have an electrician size the generator from the unit's nameplate.

The Bottom Line

Add the running watts of everything that runs at once, add 20%, and add the one biggest start-up surge for the starting watts. A fridge, sump pump, and lights fit a mid-size generator of about 3,500 running and 4,500 starting watts; add a furnace blower, a well pump, or a window AC, and you need a larger one. Whatever the size, run it outdoors at least 20 feet from the house, keep CO alarms working, and connect hardwired loads only through an electrician-installed transfer switch.

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