Generator Wattage Calculator
Choosing the right generator size requires understanding two wattage figures: running watts (the continuous load of all appliances operating simultaneously) and peak starting watts (the surge needed when the largest motor-driven appliance starts). The required generator must supply the sum of all running loads plus have enough headroom to absorb the starting surge of the most demanding appliance without stalling. This calculator sums your listed loads and adds the starting surge of the largest motor to give you the minimum generator wattage needed.
Generator sizing formula
Minimum watts = Total running watts + Starting watts of largest motor
Recommended watts = Minimum watts × 1.20 (20% headroom)
kW = Watts ÷ 1,000
Adding 20% headroom prevents running a generator at its rated limit, which reduces wear and fuel consumption. Generators run most efficiently and reliably at 50 to 80% of their rated output. Never operate a generator indoors or in an attached garage; carbon monoxide from exhaust is deadly.
Common appliance wattage reference
- Refrigerator: 150 to 400 W running, 600 to 1,200 W starting.
- Central AC (2.5 ton): 2,500 to 3,500 W running, 7,000 to 9,000 W starting.
- Window AC (10,000 BTU): 900 to 1,200 W running, 2,700 to 3,600 W starting.
- Well pump (1/2 HP): 700 to 900 W running, 2,000 to 2,700 W starting.
- Sump pump (1/3 HP): 500 to 800 W running, 1,500 to 2,400 W starting.
- LED lighting (per fixture): 10 to 20 W running, minimal starting surge.
Generator sizing: frequently asked questions
What is the difference between running watts and starting watts?
Running watts (also called rated watts) is the continuous power a generator must supply to keep an appliance running. Starting watts (surge watts) is the higher power demand that motors require for 2 to 3 seconds when they start. Most motors need 2 to 3 times their running watts to start. A generator must handle the running watts of all loads plus the starting surge of the largest motor starting at any time.
How do I calculate the starting watts for a motor appliance?
Check the appliance's nameplate or owner's manual for starting current or locked-rotor amperes (LRA). If only running watts are listed, apply a multiplier: refrigerators and freezers typically need 2 to 3x running watts to start; well pumps and air conditioners need 2 to 4x; portable tools vary widely. When in doubt, use 3x running watts as a conservative starting surge estimate.
What size generator do I need to run a whole house?
A whole-house generator typically needs 10,000 to 20,000 watts (10 to 20 kW) depending on your loads. Running a central AC (3 tons = about 3,000 running watts, 9,000 starting watts), refrigerator, some lights, and TV simultaneously requires at least a 7,500 to 10,000 watt generator. Add your critical loads and the largest single starting surge to find the minimum size.
Should I use a portable or standby generator?
Portable generators (1,000 to 12,000 W) are lower cost but must be manually started and connected, run outdoors, and need periodic refueling. Standby generators (7 kW to 20+ kW) connect permanently to your home's electrical system with an automatic transfer switch, run on natural gas or propane, and start automatically when power fails. Standby units cost several times more but provide seamless backup.
What fuel does a generator use and how long will it run?
Portable generators typically run on gasoline. A 5,000 W generator consumes about 0.5 to 0.75 gallons per hour at half load. A full 5-gallon tank runs about 7 to 10 hours. Standby generators run on natural gas (unlimited supply from utility) or propane. Always operate generators outdoors, at least 20 feet from windows, and install a battery-powered CO detector.
Official sources
- U.S. DOE Energy Saver: Backup Power and Generators.
- U.S. Consumer Product Safety Commission: Generator Safety.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.