Generator Wattage Calculator

How it works

Appliances with motors, such as refrigerators, freezers, pumps and furnace fans, need a short burst of extra power to start. That burst is often two to three times their running watts, sometimes more. Generators have two ratings to match: running (rated) watts they can supply continuously and starting (surge) watts they can supply for a few seconds.

1. Add the running watts of everything that may be on at the same time.
2. Find the appliance with the biggest jump from running to starting watts, and add that jump to the total. Motors rarely start at exactly the same moment, so this is the standard way to find peak demand.
3. Choose a generator whose running rating covers the running total with about 25 percent spare, so it works at no more than 80 percent of its rating, and whose surge rating covers the peak.

peak starting watts = total running watts + largest (starting − running) of any one appliance

recommended running watts = total running watts × 1.25, rounded up to the next 500 W

Worked example

A refrigerator, a furnace blower, a sump pump, six LED bulbs and a TV with router: 700 + 875 + 1,050 + 60 + 200 = 2,885 W running. The refrigerator has the largest starting jump, 2,200 − 700 = 1,500 W, so the peak is 4,385 W. With 25 percent spare, 2,885 × 1.25 = 3,606, which rounds up to a 4,000 W generator. It would run at about 72 percent of its rating, and its surge rating should be at least 4,385 W.

Add a 1,500 W space heater and the running total jumps to 4,385 W, the peak to 5,885 W and the recommendation to 5,500 W. Heat-producing appliances are the biggest loads in most homes.

Typical appliance wattages

These are typical published figures. Your appliances may draw more or less; check the nameplate or manual. If only amps are listed, multiply amps by volts (120 V in North America, 230 to 240 V in Australia) for watts.

Practical tips

• Stagger motor loads. Let the refrigerator start, then the sump pump, rather than switching everything on at once.
• Gas furnaces need a proper electrical connection to the generator, and many work only with clean, stable power. Inverter generators produce cleaner power for electronics and furnace controls.
• Water heaters, electric dryers, ranges and central air are 240 V loads that usually need a large generator and a transfer switch.
• Keep enough fuel for the outage you plan for, stored safely in approved containers. To see how much electricity your appliances use over a day, try the electricity cost calculator.

Carbon monoxide and electrical safety

• Never run a generator indoors, including in a garage, basement, crawlspace or shed, even with doors and windows open. Generator exhaust contains carbon monoxide (CO), a gas you cannot see or smell that can kill within minutes.
• Place the generator outdoors, well away from the house with the exhaust pointing away from doors, windows and vents. US public health guidance calls for at least 20 ft (6 m).
• Install battery or battery-backup CO alarms on every level of the home. If anyone feels dizzy, sick or weak, get to fresh air at once and call emergency services.
• Never plug a generator into a wall outlet. This "backfeeding" can energize utility lines and kill line workers or neighbors. Connect house circuits only through a transfer switch or interlock installed by a licensed electrician, as local code requires.
• Use outdoor-rated, heavy-duty extension cords sized for the load. Keep the generator dry, and let it cool before refueling.

Frequently asked questions

Last updated November 8, 2026. See how we check our numbers.