What size generator do I need for my house?

There is no single right size, because the answer depends on what you decide to keep running. The useful question is not “what size generator for a 2,000-square-foot house?” but “which loads do I need during an outage, and can I take turns with the rest?”

Step 1: Decide your tier

Most households fall into one of three tiers.

Essentials. Refrigerator, a freezer, a few lights, phone and internet, a furnace fan in winter, and a sump or well pump if you have one. This is what most people mean when they say they want to “get through” an outage.

Comfort. Essentials plus a window air conditioner or two, a microwave, a TV, and a washing machine used in turns.

Whole house. Central air conditioning, an electric water heater, an electric range or dryer, all at once, with no juggling. This is usually a permanently installed standby generator.

Step 2: Add up running watts

Using published manufacturer ranges (Champion’s chart, high end), an essentials list might look like this:

Load	Running watts	Starting watts
Refrigerator	400	1,200
Freezer	500	1,000
Furnace fan (1/2 HP)	800	1,600
Sump pump (1/2 HP)	1,050	2,150
LED lights (10 bulbs)	150	none
Modem, router, chargers	120	none
Total running	3,020

The largest extra starting demand is the sump pump (2,150 − 1,050 = 1,100 W), so the peak is about 4,120 W.

Step 3: Add headroom

Running an engine at 100% for hours is hard on it, and you will forget something. Dividing the running total by 0.8 leaves about 20% spare: 3,020 ÷ 0.8 ≈ 3,800 running watts. That points to a portable generator in roughly the 4,000 to 5,000 running-watt class with a starting rating above 4,120 W.

Step 4: Check the big 240-volt loads

Four loads change the answer dramatically:

• Central air. Champion lists a 3-ton unit at 3,000 to 3,500 running watts and 5,000 to 6,000 starting.
• Electric water heater. 3,000 to 4,500 watts with no surge.
• Electric dryer. 4,000 to 6,000 watts.
• Deep well pump over 1/2 HP. Often 240 V; 1 HP is 1,000 to 2,000 running and up to 4,000 starting.

Any one of these can double the generator you need. They also need 240-volt output and a hookup that carries it, which rules out most small inverter generators. Many homeowners handle them by taking turns: heat water for an hour, then turn it off before running the dryer.

Typical size classes

These are broad market categories, not a standard:

• About 2,000 to 2,200 W (small inverter): a refrigerator plus lights and chargers, one thing at a time.
• About 3,000 to 5,000 W (portable): essentials, including one pump or furnace fan.
• About 6,000 to 10,000 W (large portable): essentials plus comfort items, often including a 240 V well pump through a transfer switch.
• About 10 to 26 kW (home standby): much or all of the house, installed permanently with an automatic transfer switch, usually running on natural gas or propane.

Why this matters more than it used to

The U.S. Energy Information Administration reported that the average U.S. electricity customer experienced about 11 hours without power in 2024, the most in the ten years it has tracked, driven largely by hurricanes. Longer outages make fuel supply and running time as important as wattage. See the fuel run-time estimator.

Do the math for your house

Use the generator size calculator with your own appliance labels. Then read about transfer switches and interlocks before connecting anything to household wiring.

Three example households

Example 1: Essentials only, gas furnace, city water

• Refrigerator: 700 running / 2,200 starting
• Furnace blower (1/2 hp): 800 running / 2,350 starting
• Lights, router, chargers, TV: 500 running
• Microwave (taking turns): 1,000 running
• Running total: about 3,000 W. Largest surge on top: about +1,550 W.
• Suggested size: about 4,500 to 5,500 running watts.

Example 2: Rural home with a well pump

• Everything in Example 1, plus a 1 hp submersible well pump at 240 V: about 2,000 running / 4,000 to 6,000 starting
• Running total: about 5,000 W, plus the well pump's surge.
• Suggested size: about 7,500 to 9,500 running watts with a 240-volt outlet and a transfer switch.

Example 3: Near-whole-house with central AC

• Central AC (3 ton): about 3,500 running and a large starting surge (lower with a soft-start kit)
• Refrigerator, freezer, lights, electronics, microwave, furnace blower or air handler
• Usually 12,000 to 20,000+ watts. At this level, many people choose a home standby generator with load management.

These numbers are illustrations. Your appliances' labels and our calculator will give a more accurate total.

Don't forget the food clock

If you're on the fence between sizes, remember that the refrigerator doesn't have to run constantly. FoodSafety.gov, citing USDA, says a closed refrigerator keeps food safe for up to 4 hours, and a full freezer holds a safe temperature for about 48 hours. With a little load management, a smaller generator can protect your food while handling other loads in turns. See load management in an outage.

120 volts vs. 240 volts

Most household plug-in appliances run on 120 volts. Some major loads run on 240 volts: well pumps, central AC condensers, electric dryers, electric ranges, and some water heaters. To run any 240-volt load, you need:

• a generator with a 240-volt output (usually a four-prong locking L14-30 or a 50-amp outlet);
• a transfer switch or interlock installed by an electrician.

When you add up loads, keep the two 120-volt legs roughly balanced on a transfer switch. Putting all big 120-volt loads on one leg can overload that leg even when total watts look fine.

Headroom and the 80% habit

Engines last longer and handle surges better when they aren't running at their limit. A common practice is to plan for continuous loads of no more than about 80% of the generator's running rating. If your essentials total 4,000 running watts, a generator rated at 5,000 running watts or more gives you that margin. Hot weather and high altitude reduce available output, so add extra margin if either applies.

Safety and placement

Whatever size you pick, place it outdoors, at least 20 feet from windows, doors, and attached garages, as Ready.gov advises. Install carbon monoxide alarms on every level of the home, and connect through a transfer switch or interlock, never by backfeeding an outlet.