What Size Generator Do I Need? Watts, Charts and Examples (2026)

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The storm took the power out at dinner time. By the next morning the house is getting cold, the basement floor is wet because the sump pump stopped, and you’re standing in a store aisle looking at generators that say 2,200, 5,000 and 9,500 watts. Which one do you need? Here’s how to answer what size generator do I need with a pencil, a few labels and the makers’ own wattage charts.

The short answer: add up the running watts of everything you want on at the same time, then add the biggest extra jump in watts that any one motor needs to start. Pick a generator whose running watts cover the first number with some room to spare and whose starting watts cover the second. For most homes’ essentials that is 3,500 to 7,500 running watts; central air needs a standby unit.

Generator sizes at a glance

Running watts	What it handles well	Examples from makers
About 1,800–2,500 W	A fridge, lights, phones, Wi-Fi, a TV; one small appliance at a time	Honda EU2200i (1,800 W), Champion 2500W inverter (1,850 W)
About 3,500–5,000 W	Fridge, freezer, sump pump, gas-furnace blower, lights and a microwave, if you start the big loads one at a time	Champion 4500W inverter (3,500 W), Westinghouse iGen5000DFc (3,900 W)
About 7,500–10,000 W	The above plus a 240-volt well pump and more circuits through a transfer switch	Westinghouse WGen7500DFc (7,500 W), Champion 9200W tri fuel (9,200 W)
About 14–26 kW (standby)	Most or all of a house, including central air, with automatic switching	Generac Guardian 14kW to 26kW, KOHLER 20RCA and 26RCA

Watts above are gasoline ratings from each maker’s spec page. On propane or natural gas the same generator makes less: Champion rates its 9200W tri fuel model at 9,200 W on gasoline, 8,500 W on propane and 7,500 W on natural gas. Standby units are named for their propane rating; Generac’s 22kW Guardian makes 19.5 kW on natural gas.

Running watts vs starting watts

Running watts (also called rated watts) are what an appliance draws while it’s working, and what a generator can deliver hour after hour. Starting watts (surge or peak watts) are the short burst a motor needs to get moving: a fridge compressor, a pump, a furnace blower, an air conditioner. Generac describes that burst as often two to three times the running figure. It lasts only a moment, but if the generator can’t deliver it, the motor stalls or the breaker trips.

Lights, TVs, chargers, coffee makers and heaters don’t have motors, so their starting and running watts are the same. Motors are where sizing goes wrong.

How many watts your appliances use

These are the approximate figures from Honda’s generator wattage estimation guide, with Champion’s wattage chart for well pumps and central air (Champion gives ranges because models differ). Your own appliance’s label always wins.

Appliance	Running watts	Starting watts	Source
Refrigerator or freezer (ENERGY STAR)	132–192	1,200	Honda
Furnace fan, gas or oil furnace, 1/3 HP	700	1,400	Honda
Furnace fan, 1/2 HP	875	2,350	Honda
Sump pump, 1/3 HP	800	1,300	Honda
Sump pump, 1/2 HP	1,050	2,150	Honda
Well pump, 1/2 HP	750–1,000	1,500–2,000	Champion
Well pump, 1 HP	1,000–2,000	2,000–4,000	Champion
Window AC, 10,000 BTU	1,500	2,200	Honda
Central AC, 3 ton	3,000–3,500	5,000–6,000	Champion (Generac’s guide says far more; see below)
Microwave (1,000-watt oven)	1,500	1,500	Honda
Coffee maker (4 cup)	600	600	Honda
Flat-screen TV, 46 inch	190	190	Honda
Laptop	200–250	200–250	Honda
Automatic washer	1,200	1,200	Honda
Clothes dryer, electric (240 V)	5,400	6,750	Honda
Water heater, electric (240 V)	4,500	4,500	Honda
Garage door opener	720	1,420	Honda

To check your own appliances, the U.S. Department of Energy’s advice is simple: the wattage is usually stamped on the back, bottom or nameplate, and it is the maximum the appliance draws. If only amps are listed, multiply amps by volts: 120 volts for most U.S. appliances, 240 volts for big ones like dryers and electric ranges. A plug-in electricity monitor shows what something really uses.

A fridge only runs part of the time. DOE suggests treating a refrigerator as running about a third of the hours it is plugged in, and ENERGY STAR’s median top-freezer model uses 363 kWh a year, about 1 kWh a day. That matters for fuel, not for size: the generator still has to handle the 1,200-watt start every time the compressor kicks on.

How to size a generator in four steps

1. List what must run at the same time. Fridge, freezer, sump or well pump, furnace blower, lights, Wi-Fi, a phone charger, maybe a microwave. Leave out the dryer, electric oven and water heater unless you are buying a big standby unit.
2. Add up the running watts. That total is the minimum running rating.
3. Find the biggest start-up jump. For each motor, subtract running watts from starting watts. Add the largest of those jumps to your running total. That is the starting rating you need. You only add one, because you can start motors one at a time; Honda’s guide makes the same point: start the fridge and the furnace at different times and you need far less surge.
4. Leave headroom. We suggest keeping the running total at about 80% of the generator’s running rating or less. Engines last longer, fuel use is easier to plan, and there’s room for the appliance you forgot.

Also check volts. A 240-volt well pump, or any hard-wired load like a furnace, can only be fed through a transfer switch or interlock with an inlet box, installed by a licensed electrician with a permit where required. The generator needs a 240-volt outlet (usually an L14-30R or 14-50R) for that. Never plug a generator into a wall outlet to “backfeed” the house; FEMA and CPSC warn it can kill utility workers and neighbors.

Worked examples

1. Fridge, lights and Wi-Fi: about 2,000 watts

Fridge (192 W) + ten LED bulbs (about 100 W; check the bulbs) + flat-screen TV (190 W) + router and modem (about 30 W; check their labels) + a laptop (250 W) = about 760 running watts. The fridge’s start-up jump is about 1,000 W (1,200 − 192), so you need about 1,760 starting watts. A 2,200-watt inverter generator such as the Honda EU2200i (1,800 W running, 2,200 W starting) covers it, with room left for a coffee maker once the fridge is running.

2. Winter essentials with a sump pump: about 3,500–5,000 watts

Fridge (192 W) + 1/3 HP gas-furnace blower (700 W) + 1/2 HP sump pump (1,050 W) + lights, TV and Wi-Fi (320 W) = 2,262 running watts. The biggest jump is the sump pump: 2,150 − 1,050 = 1,100 W, so you need about 3,362 starting watts. Add a microwave (1,500 W) and the running total climbs to 3,762 W.

Without the microwave, a 3,500-watt-running inverter like the Champion 4500W (3,500 W running, 4,500 W starting) does the job with headroom. With the microwave, 3,762 W is close to the limit of even the Westinghouse iGen5000DFc (3,900 W running, 5,000 W starting): it works if you heat food only while the pump is off, but for real breathing room choose about 5,000 running watts or an open-frame model around 7,500 W. The furnace is hard-wired, so it needs a transfer switch or interlock installed by a licensed electrician.

3. A home on a well: about 7,500 watts and 240 volts

Take example 2 (2,262 running watts) and add a 1 HP well pump. Using the top of Champion’s range (2,000 W running, 4,000 W starting), you reach 4,262 running watts and need about 6,262 starting watts. Franklin Electric, which makes many of the submersible motors in U.S. wells, sets a minimum generator of 4 kW (5 kVA) for a 1 HP motor, and 50% more for best starting of a 2-wire motor, so about 6 kW. Both roads lead to a generator with roughly 7,500 running watts and a 240-volt outlet, like the Westinghouse WGen7500DFc or Champion 9200W tri fuel. Franklin also says to start the generator before turning on the pump and to turn the pump off before stopping the generator.

4. Central air conditioning: plan on a standby generator

Here the charts disagree. Champion’s chart lists a 3-ton central AC at 3,000–3,500 running and 5,000–6,000 starting watts. Generac’s sizing guide says a 3-ton unit typically needs about 3.5–5 kW running but can demand up to 15–18 kW to start the compressor. The difference is the compressor’s locked-rotor amps (LRA), printed on the outdoor unit’s nameplate. Multiply LRA by 240 volts for a rough idea of the start-up demand; that’s why central air is usually a standby-generator job. Generac’s guide suggests 18 to 22 kW as the sweet spot for a 2,000 sq. ft. home, and smaller units paired with load management that switches big loads on and off. Have a licensed dealer do a load calculation; don’t guess with a portable generator.

Fuel: the cost of going bigger

A bigger generator burns more fuel, even at the same load if it’s a conventional model. Westinghouse rates the iGen5000DFc at up to 18 hours on 3.4 gallons at 25% load; running around the clock at that load would take about 4.5 gallons a day by our math. Champion rates its 9200W tri fuel at 10 hours on 8.5 gallons at half load, about 20 gallons a day. Running 24 hours a day you’ll be refueling often. Turn it off and let it cool before every refill; Ready.gov warns that fuel spilled on hot engine parts can ignite. Store gasoline in approved cans away from the house, treat it with a stabilizer (STA-BIL says its Storage formula keeps gas fresh up to 24 months), and use it up and replace it in rotation.

Carbon monoxide: the part that keeps your family alive

A portable generator’s exhaust can kill in minutes. CPSC says nearly 100 people in the U.S. die each year from carbon monoxide from portable generators, and that one generator can produce as much CO as hundreds of cars. Follow CPSC and CDC rules every time:

• Run it outdoors only, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents.
• Never in the house, garage, basement, crawlspace, shed or porch, even with the door or windows open.
• Put battery-powered or battery-backup CO alarms on every level and outside sleeping areas, and test them.
• Choose a model with a CO shut-off: Honda calls it CO-MINDER, Champion CO Shield, Generac COsense, Westinghouse a CO Sensor and DuroMax CO Alert. It does not replace alarms or the 20-foot rule.
• If anyone feels dizzy, sick or weak, get everyone outside to fresh air and call 911.

Generators by size

Prices change often. The prices below are what we saw at the maker or a major retailer in October 2026; check the current price before you buy.

Model	Running / starting (gas)	240 V outlet	Weight	Price seen
Honda EU2200i	1,800 / 2,200 W	No	47.4 lb	about $1,199 (Honda MSRP)
Champion 4500W Dual Fuel Inverter	3,500 / 4,500 W	No	99.2 lb	check price
Westinghouse iGen5000DFc	3,900 / 5,000 W	Only with the parallel kit	105.8 lb	about $1,299 (maker’s list price)
Westinghouse WGen7500DFc	7,500 / 9,500 W	L14-30R	202.1 lb	about $1,299 (maker’s list price)
Champion 9200W Tri Fuel	9,200 / 11,500 W	L14-30R, 14-50R	216 lb	check price
Generac Guardian 22kW	22 kW LP / 19.5 kW NG	Hard-wired	–	from $6,119 MSRP, unit only

Westinghouse lists its prices on its own store but showed both models as unavailable to buy there in October 2026. The Generac price is the unit only: the transfer switch, installation, gas line, concrete pad and permits are extra, and the Guardian is sold and installed through Generac dealers, not shipped to your door. Get two or three quotes.

Check the Westinghouse WGen7500DFc price on Amazon

See it at Generac

Keep exploring

Start at our home generators hub, then compare the best generators for a power outage, inverter generators and dual fuel generators. Hooking a generator to your panel is covered in our transfer switch guide, and the full safety routine in how to run a generator safely. If you’d rather have no engine at all, see home battery backup, portable power stations and what size power station you need. Fuels are explained on our propane, natural gas and petroleum pages, and every generator article is in the generators category.

Generator sizing questions

What size generator do I need to run a refrigerator?

Honda’s guide puts an ENERGY STAR fridge at 132–192 running watts and about 1,200 watts to start. Any generator with at least 2,000 starting watts will start it, and a 2,200-watt inverter runs a fridge plus lights, Wi-Fi and phone chargers. Older or larger fridges can need more, so check the label.

What size generator runs a whole house?

Running everything, including central air, an electric range and a dryer, usually takes a standby generator. Generac’s guide points a 2,000 sq. ft. home toward 18–22 kW. A 7,500–10,000-watt portable can run most essentials through a transfer switch, but not all at once. A dealer’s load calculation is the only reliable number.

Will a 3,500-watt generator run a sump pump?

Usually, yes. Honda lists a 1/2 HP sump pump at 1,050 running and 2,150 starting watts, and a 1/3 HP pump at 800 and 1,300. A generator with 3,500 running and 4,500 starting watts starts either pump with room for a fridge and lights. Plug the pump straight into the generator with a heavy-duty outdoor cord.

What size generator do I need for a well pump?

Franklin Electric’s minimum is 2 kW for a 1/2 HP motor and 4 kW for 1 HP, with 50% more for best starting of 2-wire motors. Most well pumps are hard-wired, many at 240 volts, so you also need a generator with a 240-volt outlet and a transfer switch installed by a licensed electrician.

Can a portable generator run central air?

Rarely well. Generac says a 3-ton unit can need 15–18 kW for a moment to start the compressor, more than most portables make. Some large portables may start a small system, but check the compressor’s LRA on the outdoor unit and ask an HVAC contractor or generator dealer before trying. A window AC in one room is the practical portable choice.

Is it bad to buy a generator that’s too big?

Not dangerous, but costly. A bigger conventional generator burns more fuel even at light load, because its engine runs at a fixed speed, and it’s heavier to move and harder to feed for days. Size for what you’ll actually run, plus about 20% headroom, and start big motors one at a time.