How House Works

Generator size calculator

Tick what has to run at the same time. The calculator adds up the running watts, then adds the largest extra starting surge of a single item.

By How House Works EditorialBased on Manufacturer wattage chartsChecked against sources

Appliances, how many run at once, running watts and extra starting watts
RunApplianceQtyRunning WExtra starting W
Basics
Refrigerator/freezer
Furnace fan blower, 1/2 HP
Sump pump, 1/2 HP
Sump pump, 1/3 HP
Lights, 4 × 75 W incandescentLED bulbs: Champion lists 8–15 W per bulb.
Internet router/modemChampion lists 10–20 W; the top of the range is used.
Cell phone charger
Laptop computer
CPAP with heated humidifierChampion lists 60–120 W (30–60 W without heat); the top of the range is used.
Television, LED
Water
Well pump, 1/2 HPSubmersible pumps: see the Franklin Electric minimums below.
Well pump, 1/3 HP
Electric water heaterChampion lists 3,000–4,500 W. Check the element wattage on the rating plate.
Kitchen
Microwave, 1,000 W
Coffee maker
Electric range, one 8 in. element
Toaster
Dishwasher
Laundry
Washing machine
Clothes dryer, electricChampion lists 4,000–6,000 W with no extra starting watts; the top of the range is used.
Heating and cooling
Window air conditioner, 12,000 BTUGenerac's GP owner's manual lists 1,700 W for the same size. Use the unit's rating plate if you can.
Central air conditioner, 24,000 BTUCompressor starting draw varies a lot by unit. See the note on central AC below.
Heat pumpGenerac does not state the size of the heat pump in this row.
Space heaterChampion lists 750–1,500 W; the top of the range is used.
Other
Garage door opener, 1/2 HP

"Extra starting W" is the surge on top of running watts, the way Generac, Champion, Westinghouse and Briggs & Stratton list it. Leave it at 0 for things without a motor. If an appliance's rating plate or manual gives its wattage, use that number instead of the chart value.

How to read the result

Running watts are the continuous power needed to keep everything on. Starting watts are the extra power a motor needs for two to three seconds to get going (Briggs & Stratton). A portable generator is rated for both, and its ratings should meet or beat both numbers above.

The peak counts only one starting surge because the manufacturers' method assumes motors start one at a time. Generac's wattage table notes that its totals assume "intermittent starting of devices", and Champion's manual says to plug in and switch on items one by one, letting the engine settle in between. Honda's example shows the payoff: started separately, a refrigerator and a furnace need 1,600 W instead of 3,200 W.

The margin setting is optional. In an article on sizing a generator for tailgating, Champion suggests adding 20% and says not to run a generator at 100% of its rating for long periods; Briggs & Stratton suggests 20–30%. The Generac, Honda and Westinghouse documents used here give no percentage. Honda's manual says not to exceed the rated (running) power for continuous use.

Standby generators: check the rating for your fuel

A home standby generator makes less power on natural gas than on propane (LP). In these spec sheets, the size in the model name is the LP rating.

Model (spec sheet)LPNatural gas
Generac Guardian 22 kW (Rev. F, 2022)22,000 W19,500 W
Generac air-cooled 22 kW (Rev. D, 2026)22 kW21 kW
Kohler 20RCA (G4-272, 2023)20 kW18 kW

Output also drops with altitude and heat. Generac's spec sheets give about 3.5% less for each 1,000 ft above sea level and about 1% less for each 10 °F above 60 °F. Kohler's sheet says to derate 4% per 1,000 ft above 500 ft and 2% per 10 °F above 60 °F. Generac describes its standby rating as having no overload capability.

Central air conditioners

Treat the central AC row as a rough starting point. Generac's sizing guide says a 3-ton central air conditioner typically runs on about 3.5 to 5 kW but can need up to 15 to 18 kW to start the compressor. Generac's portable brochure lists a 24,000 BTU central unit at 3,800 running and 4,950 extra starting watts. If the generator must start a central AC, have the generator dealer or a qualified professional size it from the air conditioner's own data.

Well pumps

For submersible well pumps, Franklin Electric publishes minimum generator sizes by pump motor rating. The table covers the pump motor only, so other loads come on top of it.

Pump motor3-wire or 3-phase motor2-wire motor
1/3 HP1.5 kW (1.9 kVA)2.25 kW (2.85 kVA)
1/2 HP2 kW (2.5 kVA)3 kW (3.75 kVA)
3/4 HP3 kW (3.8 kVA)4.5 kW (5.7 kVA)
1 HP4 kW (5.0 kVA)6 kW (7.5 kVA)
1.5 HP5 kW (6.25 kVA)7.5 kW (9.375 kVA)

Franklin Electric, AIM Manual (2026), printed p. 7. Franklin also says to start the generator before turning on the pump, and to turn the pump off before shutting the generator down (printed p. 6).

Before you connect a generator

Methodology

The calculator follows the method Generac, Champion and Briggs & Stratton describe for sizing a generator:

  1. Running watts = sum of running watts × quantity for every ticked item.
  2. Peak watts = running watts + the largest single "extra starting watts" value among the ticked items.
  3. With a margin, both numbers are multiplied by 1 + margin, as in Champion's example of a 20% margin.
  4. The recommended ratings ("Look for a generator rated at least…") are rounded up to the next 100 W. That rounding is our choice; the running and peak totals are shown as calculated.

The main source of defaults is Generac's GP series brochure, whose table lists running watts and additional starting watts. Rows it doesn't cover come from the Westinghouse WGen12000 manual and Champion's flyer, which use the same additional-starting convention, and from Champion's web chart and a Generac-hosted worksheet for items with no starting surge. Where a source gives a range, the top of the range is used and the row says so.

Default values

ApplianceRunning WExtra starting WSource
Refrigerator/freezer7002,200Generac GP brochure, p. 3
Furnace fan blower, 1/2 HP8752,300Generac GP brochure, p. 3
Sump pump, 1/2 HP1,0502,150Generac GP brochure, p. 3
Sump pump, 1/3 HP8001,300Westinghouse WGen12000 manual, p. 18
Lights, 4 × 75 W incandescent3000Westinghouse WGen12000 manual, p. 18
Internet router/modem200Champion wattage chart (web)
Cell phone charger100Generac-hosted wattage worksheet
Laptop computer750Generac-hosted wattage worksheet
CPAP with heated humidifier1200Champion wattage chart (web)
Television, LED800Champion wattage flyer
Well pump, 1/2 HP1,0002,100Generac GP brochure, p. 3
Well pump, 1/3 HP1,0002,000Westinghouse WGen12000 manual, p. 18
Electric water heater2,0001,000Generac GP brochure, p. 3
Microwave, 1,000 W1,0000Generac GP brochure, p. 3
Coffee maker1,5000Generac GP brochure, p. 3
Electric range, one 8 in. element2,1000Generac GP brochure, p. 3
Toaster9000Champion wattage flyer
Dishwasher1,2001,500Champion wattage flyer
Washing machine1,1502,300Generac GP brochure, p. 3
Clothes dryer, electric6,0000Champion wattage chart (web)
Window air conditioner, 12,000 BTU3,2503,950Generac GP brochure, p. 3
Central air conditioner, 24,000 BTU3,8004,950Generac GP brochure, p. 3
Heat pump4,7004,500Generac GP brochure, p. 3
Space heater1,5000Champion wattage chart (web)
Garage door opener, 1/2 HP8752,350Generac GP brochure, p. 3

How much the charts disagree

Manufacturers' charts differ, sometimes by a factor of two. Champion's flyer lists a refrigerator at 400 running watts where Generac lists 700. Generac's brochure gives a 12,000 BTU window air conditioner 3,250 running watts, while Generac's GP owner's manual lists 1,700 for the same size. That is why every value here can be edited.

What the calculator does not account for

  • Altitude, heat and fuel type, which lower a generator's output (see the standby section above).
  • Whether a 120 V load sits on one leg of a 240 V generator, and whether the generator has a 240 V outlet.
  • Soft starters, load managers and other devices that change how motors start.
  • How long the generator will run and how much fuel it will use.
  • The motor-starting ability of a specific standby unit. That is a job for the dealer or electrician.

Sources

  1. Generac Power Systems. GP Series Portable Generators 3600–8000 watt brochure (wattage table). February 2023, p. 3. Accessed Oct 12, 2026.
  2. Westinghouse Outdoor Power Equipment. WGen12000 Portable Generator User Manual (Wattage Reference). 2021, pp. 17–18. Accessed Oct 12, 2026.
  3. Champion Power Equipment. Choosing a Generator: Quick Reference Wattage (flyer). p. 1. Accessed Oct 12, 2026.
  4. Champion Power Equipment. Generator Wattage Chart: Common Appliances & Tools. April 17, 2026, Home & Backup Power table. Accessed Oct 12, 2026.
  5. CHP Inc. / Generac. Generator Wattage Worksheet (DR Power Equipment, hosted by Generac). 2018, p. 1. Accessed Oct 12, 2026.
  6. Briggs & Stratton. Choosing a Portable Generator. January 13, 2026, Determine the Size You Need. Accessed Oct 12, 2026.
  7. American Honda Motor Co.. Generator Power Management. Accessed Oct 12, 2026.
  8. Champion Power Equipment. Operator's Manual, Model 100554. February 14, 2022, pp. 4, 23. Accessed Oct 12, 2026.
  9. Champion Power Equipment. What Size Generator Do I Need for Tailgating?. September 17, 2026. Accessed Oct 12, 2026.
  10. Briggs & Stratton Energy Solutions. Motor Starting 101. August 19, 2024. Accessed Oct 12, 2026.
  11. Honda Motor Co.. EU7000iS Owner's Manual. pp. 62, 71 (printed). Accessed Oct 12, 2026.
  12. Generac Power Systems. What Size Generator Do I Need?. Accessed Oct 12, 2026.
  13. Generac Power Systems. GP Series Portable Generator Owner's Manual (GP15000E/GP17500E). 2019, printed pp. 12–13. Accessed Oct 12, 2026.
  14. Generac Power Systems. Guardian Series 20/22/24 kW standby generator spec sheet. A0000937814 Rev. F, September 28, 2022, p. 4. Accessed Oct 12, 2026.
  15. Generac Power Systems. Air-Cooled Standby Generator 22–28 kW spec sheet. A0005151077 Rev. D, August 13, 2026, p. 4. Accessed Oct 12, 2026.
  16. Kohler Co.. Models 20RCA(L) spec sheet G4-272. July 2023, p. 1. Accessed Oct 12, 2026.
  17. Franklin Electric. AIM Manual, 60 Hz Submersible Systems (M1311). 2026 edition, printed pp. 6–7. Accessed Oct 12, 2026.
  18. U.S. Consumer Product Safety Commission. CPSC Warns of Generator, Carbon Monoxide and Fire Hazards Ahead of Hurricane Season. May 27, 2026. Accessed Oct 12, 2026.
  19. U.S. Consumer Product Safety Commission. Carbon Monoxide Information Center. Accessed Oct 12, 2026.
  20. U.S. Department of Labor, OSHA. Using Portable Generators Safely (fact sheet). 2005. Accessed Oct 12, 2026.