Generator Size Calculator: What Size Generator Do I Need?

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Add the running watts of everything you'll run at once, plus the largest single starting watts. A refrigerator, furnace fan, sump pump, lights and TV need at least 3,504 running and 6,084 starting watts with a 20% margin.

What size generator do you need?

Pick what must run at the same time during an outage. Add everything else as other items: their total running watts, and the starting watts of the one that needs the most to start (0 if none has a motor). The safety margin starts at 20%, the low end of the 20 to 30% Briggs & Stratton suggests.

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Estimates only. Appliance watts are the top of each range in Champion Power Equipment's wattage chart; for exact figures, pick None and enter the watts from your own labels as other items. Run a portable generator outdoors only, at least 20 ft from the house with the exhaust away from windows, doors, and vents (CPSC), and keep CO alarms inside. Connect it to home wiring only through a transfer switch installed by a licensed electrician; never plug it into a wall outlet.

Our generator size calculator follows the method Champion Power Equipment and Briggs & Stratton publish, with appliance watts from Champion's 2026 chart. Your appliance labels give exact figures, and a licensed electrician should connect any generator to your home's wiring.

What to enter in the generator size calculator

  1. Decide what must run at the same time. Briggs & Stratton's first step is "Select the items you wish to power at the same time."
  2. Pick the appliances in the lists. The calculator uses the top of each range in Champion's chart. Choose None for anything you don't have; enter a second unit as an other item.
  3. Add up the running watts of everything else. The Department of Energy (DOE) says wattage "is usually stamped on the bottom or back of the appliance, or on its nameplate." If only amps are listed, multiply by the volts: 120 for most U.S. appliances, 240 for large ones such as clothes dryers (12.5 A × 120 V = 1,500 W). The default 670 W is 10 LED bulbs, a TV, a modem and a phone charger at the top of Champion's ranges (150 + 400 + 20 + 100 W). The electricity cost calculator turns the same label watts into kWh per day.
  4. Enter the largest starting watts among those other items. Only motors need extra, such as a washer, a freezer or a central air conditioner. For lights, TVs, microwaves and heaters, Champion's chart says "Size based on running watts only." Enter 0 if nothing else has a motor.
  5. Set the safety margin. It starts at 20%, the low end of Briggs & Stratton's advice to "add a safety margin of 20-30%."

The generator size formula

  • Running watts = the running watts of everything on at once, added up
  • Peak watts = running watts + the largest single starting wattage
  • Minimum ratings = running and peak watts × (1 + margin ÷ 100), rounded up to whole watts

Champion's guide says to "Identify the highest starting wattage among your appliances" and "Add that starting wattage to your total running watts to determine your peak power need." Briggs & Stratton puts it the same way: "Select the ONE ITEM with the highest number of additional starting watts. Take this ONE NUMBER, add it to your TOTAL RUNNING WATTS."

Worked example (the calculator's default values)

A refrigerator, a 1/2 HP furnace blower fan, a 1/2 HP sump pump and 670 W of other items, with a 20% margin:

  1. Running: 400 + 800 + 1,050 + 670 = 2,920 W.
  2. Starting watts: refrigerator 1,200, furnace fan 1,600, sump pump 2,150, the largest.
  3. Peak: 2,920 + 2,150 = 5,070 W.
  4. With 20%: 2,920 × 1.2 = 3,504 running watts, and 5,070 × 1.2 = 6,084 starting watts.

The calculator shows: "Look for a generator rated at least 3,504 running watts and 6,084 starting watts (3.504 and 6.084 kW)." and "Running: 1 refrigerator (400 W) + 1/2 HP furnace fan (800 W) + 1/2 HP sump pump (1,050 W) + other items (670 W) = 2,920 W. Peak: 2,920 W running + 2,150 W to start the 1/2 HP sump pump (the largest starting load) = 5,070 W. With a 20% safety margin (x 1.2, rounded up to whole watts): 3,504 W running and 6,084 W starting."

Why add only the largest starting load?

The surge is brief: Briggs & Stratton describes starting watts as "extra watts needed for two-to-three seconds to start motor-driven products like a refrigerator or circular saw." If motors start one at a time, only one surge sits on top of the running load; adding every starting figure in the example would give 7,870 W instead of 5,070 W. Honda's tip keeps it that way: "Start appliances with high starting wattage requirements at different times."

Generator size for a refrigerator, furnace fan, sump pump and lights: 3,504 running and 6,084 starting wattsA table lists running and starting watts: refrigerator 400 and 1,200, 1/2 HP furnace fan 800 and 1,600, 1/2 HP sump pump 1,050 and 2,150 (the largest start), and lights, TV, modem and charger 670 with no starting watts. The running watts add up to 2,920. A second bar adds only the largest start, the sump pump's 2,150 watts, for a 5,070 watt peak. A 20 percent safety margin raises them to 3,504 running and 6,084 starting watts. A third bar shows that adding every starting figure would give 7,870 watts, which the method does not do.GENERATOR SIZE: RUNNING WATTS + THE LARGEST STARTWhat runs at onceRunningStartingRefrigerator400 W1,200 WFurnace fan (1/2 HP)800 W1,600 WSump pump (1/2 HP)1,050 W2,150 WlargestLights, TV, modem, charger670 WnoneTotal running2,920 WRunning: everything on at once2,920 W → 3,504 W with 20%Peak: running + only the largest start (the sump pump)+2,150+20%5,070 W → 6,084 WNot the rule: every starting figure added7,870 WRunning: 400 + 800 + 1,050 + 670 = 2,920 WPeak: 2,920 + 2,150 to start the sump pump = 5,070 WWith a 20% margin: 2,920 × 1.2 = 3,504 W runningand 5,070 × 1.2 = 6,084 W starting (3.504 and 6.084 kW)
Add the running watts of everything on at once (2,920 W), then only the largest single starting load, here the sump pump's 2,150 W, for a 5,070 W peak. A 20% safety margin gives 3,504 running and 6,084 starting watts. Adding every starting figure (7,870 W) is not how the method works.

Generator wattage chart for common household items

Running and starting watts from Champion Power Equipment's chart. Our calculator uses the top of each range for its list items.

ApplianceRunning wattsStarting watts
Refrigerator150 to 400800 to 1,200
Freezer100 to 500500 to 1,000
Furnace fan (1/2 HP)300 to 800800 to 1,600
Sump pump (1/3 HP)500 to 8001,000 to 1,600
Sump pump (1/2 HP)800 to 1,0501,300 to 2,150
Well pump (1/2 HP)750 to 1,0001,500 to 2,000
Well pump (1 HP)1,000 to 2,0002,000 to 4,000
Window AC (5,000 BTU)450 to 600900 to 1,200
Window AC (10,000 BTU)900 to 1,2001,800 to 2,400
Central AC (3 ton)3,000 to 3,5005,000 to 6,000
Washing machine500 to 1,0001,000 to 2,300
Microwave600 to 1,200None extra
Space heater750 to 1,500None extra
Electric water heater3,000 to 4,500None extra
TV (32 to 55 in)80 to 400None extra
LED bulb8 to 15None extra
Router or modem10 to 20None extra
Phone or laptop charger20 to 100None extra

Models differ: Honda's wattage calculator lists an ENERGY STAR refrigerator at 192 W running, so your own label beats any chart.

What size generator to run a house

It depends on what you run: Generac's support team says sizing a home standby generator is about electrical load, not square footage. From the calculator, with a 20% margin:

What runs at onceRunning wattsPeak wattsLook for (running / starting)
Refrigerator only4001,600480 / 1,920
Refrigerator + 670 W of lights and electronics1,0702,2701,284 / 2,724
Refrigerator + 10,000 BTU window AC + 670 W2,2704,6702,724 / 5,604
Essentials: refrigerator, 1/2 HP furnace fan, 1/2 HP sump pump, 670 W2,9205,0703,504 / 6,084
Essentials + 1/2 HP well pump3,9206,0704,704 / 7,284
Essentials + 1 HP well pump4,9208,9205,904 / 10,704
Essentials + 3-ton central AC + electric water heater10,92016,92013,104 / 20,304

For the last row, enter the central AC's 3,500 running watts and the water heater's 4,500 W as other items (8,670 W with the 670 W), and the AC's 6,000 starting watts as the largest. Champion describes its portable generators as available from 1,000 to 22,000+ watts and its home standby units from 8,500 to 26,000+ watts, so the two overlap. For a standby unit, Generac is plain: "Professional installation is required, not optional."

What size generator to run a refrigerator

At the top of Champion's range, a refrigerator runs at 400 W and needs 1,200 W more to start, a 1,600 W peak. With the 20% margin, look for at least 480 running and 1,920 starting watts. A refrigerator and a freezer (an other item: 500 running, 1,000 starting) come to 1,080 and 2,520.

Generator safety: carbon monoxide and transfer switches

  • Outdoors only. The Consumer Product Safety Commission (CPSC) says: "Never use a portable generator inside homes, garages, basements, crawlspaces, sheds or other enclosed areas." Run it "outdoors only, at least 20 feet from homes and buildings," with the exhaust pointed away from windows, doors and vents.
  • CO alarms inside. CPSC recommends battery-operated CO alarms, or alarms with battery backup, outside sleeping areas. The same alarms guard against a gas furnace; the furnace size calculator guide covers CPSC's advice for them.
  • A transfer switch, installed by an electrician. OSHA: "Never attach a generator directly to the electrical system of a structure (home, office, trailer, etc.) unless a qualified electrician has properly installed the generator with a transfer switch."
  • Never backfeed. CPSC: "NEVER try to power the house wiring by plugging the generator into a wall outlet." University of Illinois Extension explains that a transfer switch keeps generator power off utility lines, "when it could be dangerous to a lineman or others near downed power lines, a process known as 'back feed.'"
  • Fuel. OSHA: "Before refueling, shut down the generator and allow it to cool."

Tips and common mistakes

  • Running at the limit. OSHA: "Do not overload a generator; this can lead to overheating which can create a fire hazard." Generac's sizing paper for mobile generators puts the optimum at "70-85% of the machine's rated capacity." A 20% margin keeps the running load at 83% of the minimum rating.
  • Forgetting outlet limits. Each outlet on a portable generator has its own limit, and Generac calls knowing them "key to choosing the right size generator."
  • Starting everything at once. Stagger motor starts. Honda: "If an appliance is only being used occasionally for short periods, run it by itself."
  • Guessing the window unit. Size it to the room first with the AC size calculator, then pick its BTU class here.

Frequently asked questions

What size generator do I need to run a house?

Add up what runs at once, not the floor area. Essentials (a refrigerator, a 1/2 HP furnace fan, a 1/2 HP sump pump, lights and a TV) need at least 3,504 running and 6,084 starting watts with a 20% margin. Adding a 3-ton central AC and an electric water heater raises that to 13,104 and 20,304 watts.

What size generator do I need to run a refrigerator?

At the top of Champion's chart, a refrigerator runs at 400 W and needs 1,200 W to start, a 1,600 W peak. With a 20% margin, look for at least 480 running and 1,920 starting watts. Your refrigerator's label gives its own figure.

How do I calculate what size generator I need?

Add the running watts of everything you will run at the same time, then add the highest single starting wattage to get the peak. Add a safety margin (Briggs & Stratton suggests 20 to 30%) to both.

What size portable generator do I need for a power outage?

For a refrigerator, a furnace blower fan, a 1/2 HP sump pump, lights, a TV and a modem, look for at least 3,504 running watts and 6,084 starting watts. A 1 HP well pump raises that to 5,904 and 10,704. Run it outdoors, at least 20 feet from the house.

What size generator do I need for an RV or camper?

Size it for the air conditioner. Champion lists a 13,500 BTU RV air conditioner at up to 2,000 running and 3,500 starting watts, and an RV refrigerator at up to 600 running watts. Entered as other items, that is a 6,100 W peak, or 3,120 running and 7,320 starting watts with a 20% margin.

What is the difference between running watts and starting watts?

Running watts keep an appliance going; starting watts are the extra a motor needs to start. Briggs & Stratton says a start takes approximately two to three times the rated running wattage, for two to three seconds. Lights, TVs and heaters need no extra starting watts.

How much bigger than my load should a generator be?

Briggs & Stratton suggests a safety margin of 20 to 30% on top of the total wattage. The calculator starts at 20%: the default 2,920 W running load becomes a 3,504 W minimum, so the generator would run at about 83% of that rating.

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