Prep

Generator size calculator

Pick what you'd run at the same time. You'll see the watts you need, and whether a generator you own or are looking at can cover it.

A generator used indoors can kill you in minutes. Its exhaust contains carbon monoxide (CO), a poison you can't see or smell.

  • Run it outdoors only, more than 20 feet from windows, doors and vents (CDC), with the exhaust pointing away from the home and other buildings (CPSC).
  • Never run it inside a home, garage, basement, crawlspace or shed, or on a porch, even with doors and windows open (CPSC).
  • Install battery or battery backup CO alarms outside sleeping areas and on each floor, and test them monthly (CPSC).
  • If you feel sick, dizzy or weak, or a CO alarm sounds, get outside immediately, then call 911 (CPSC).

Never plug a generator into a wall outlet. This is called backfeeding, and it can electrocute utility workers and neighbors (CPSC). To connect one to your home's wiring, have a licensed electrician install a transfer switch.

What would you run at the same time?

Typical figures from Generac's chart. Check the plate on your appliance and change them.

Plate shows amps? Amps × 120 volts gives watts. For a medical device, use its own label and talk to your medical provider and equipment supplier.

Have a generator in mind?

From preppantry.app. Checked 2 November 2026. Independent tool, not official advice.

How it works

The method. Add up the running watts of everything you want to power at the same time. Then add the single highest starting figure. Motors need a short burst of extra power to start, and only one is assumed to start at a time. This is the method in Generac's brochure, and its figures assume devices start one at a time, so don't switch everything on at once.

Example: a refrigerator (700 running, 2,200 starting) and a sump pump (1,050 running, 2,150 starting). That's 1,750 running watts. The highest starting figure is 2,200. So you'd need 3,950 starting watts, and at least 1,750 running watts.

The figures are typical, not yours. They come from Generac's chart, which calls them approximate, and we've listed its household rows. Real appliances vary a lot: refrigerators alone range from about 132 to 700 running watts across the sources we checked. Check the plate on your appliance and type its figures in. If the plate shows amps instead of watts, amps × volts gives you watts, and most US household outlets are 120 volts.

What's not included. We don't add a safety margin, because we have no official figure for one. We also don't size whole-home standby systems, which an electrician should work out. A generator needs fuel, and we don't give fuel storage advice, because there are no official household figures. Follow your owner's manual.

Connecting it. Plug appliances into the generator with heavy-duty outdoor extension cords rated for at least the total wattage of what's on them. Check that each cord has no cuts and that the plug has all three prongs. To power anything through your home's wiring, a licensed electrician needs to install a transfer switch first. OSHA says never to attach a generator directly to a building's electrical system without one, because it can energize wiring for great distances and put utility workers and others at risk.

In the rain. Keep the generator dry. If you have to run it when it's wet outside, CPSC says to use an open, canopy-like structure on a dry surface, where water can't reach it or puddle under it, without putting it indoors or near openings to any building. Dry your hands before touching it.

Refueling. Turn it off and let it cool first, because gasoline spilled on hot engine parts could ignite. Never store fuel in your home, and don't store it near a fuel-burning appliance such as a natural gas water heater in a garage.

Common questions

What can a 3,000 watt generator run?

It depends on whether 3,000 is its running or starting rating. On Generac's typical figures, a 3,000 running watt generator with enough starting power could run a refrigerator and a sump pump, which is 1,750 running watts and 3,950 starting watts. Enter your generator's ratings above to check your own list, and use the plate figures for your appliances.

What's the difference between running and starting watts?

Running watts are what a device uses once it's going. Starting watts are the extra burst a motor needs for a moment as it starts. Refrigerators, pumps and air conditioners all need one. Heating devices like coffee makers don't.

How far from the house should a generator be?

The CDC says outdoors, more than 20 feet from windows, doors and vents. CPSC says at least 20 feet from the house, with the exhaust pointing away from the home and other buildings. Ready.gov adds attached garages to the list. Never run one on a porch or in a garage, even with the door open.

Can I plug a generator into my house?

Not into a wall outlet. CPSC warns that this is called backfeeding, and that it's extremely dangerous: it can electrocute utility workers and neighbors served by the same transformer, and it bypasses some of your home's circuit protection. A licensed electrician can install a transfer switch, which is the safe way to connect one.

Does a CO alarm make it safe to run a generator indoors?

No. Never run one indoors, with or without an alarm. CPSC says deadly levels can build up quickly even with fans or open doors and windows, and can linger for hours after the generator has shut off. Alarms are a backup. CPSC also suggests asking retailers for a model that shuts off automatically when high CO levels are present.

Can a generator run a medical device?

Check the device's own label, and talk to your medical provider and equipment supplier about a power outage plan. We don't list figures for medical devices. Ready.gov advises having alternate plans for anything that depends on power.

Sources

Checked on 2 November 2026. This is an independent tool and not official advice. See sources and method.

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