What Size Generator Do I Need for My House?

1. Decide what must keep running

For most homes the essentials are the refrigerator and freezer, the sump pump, the furnace fan in winter, a few lights, internet and phone chargers. A well pump, a window AC or a medical device may be essential for you. Central air conditioning and electric water heaters need a much larger generator.

2. Add the running watts

Running watts are what each appliance uses continuously. Typical values from Honda Power Equipment’s wattage guide: refrigerator 700 W, 1/2 HP sump pump 1,000 W, 1/3 HP furnace fan 1,000 W. Add everything that may run at the same time.

3. Cover the biggest starting surge

Motors need a short burst of extra power to start — a refrigerator rated 700 W running can need about 2,200 W for a moment. You do not need to add every surge, because motors rarely start at exactly the same time: add only the largest single surge (starting watts minus running watts) to the running total.

Example: refrigerator, sump pump, furnace fan, TV, computer and 200 W of lights run at 3,170 W. The refrigerator’s surge adds 1,500 W, for 4,670 starting watts. With a 10% margin, a generator of about 5,500 starting watts fits. The generator size calculator does this for you.

4. Choose the type

• Inverter generators are quieter, more fuel-efficient at light loads and produce clean power for electronics.
• Conventional portable generators give more watts for the money.
• Standby generators are installed permanently, start automatically and can power the whole house.
• Battery backup is silent and fume-free; size it with the battery calculator.

Safety essentials

Run portable generators outdoors only, well away from doors and windows, and install carbon monoxide alarms. Never plug a generator into a wall outlet (“backfeeding”) — it can electrocute utility workers. Connect to home circuits only through a transfer switch or interlock installed by a licensed electrician. To check circuit loads, use the watts to amps calculator.