What size generator do you need to run a house?
Short answer
For the essentials, most homes need a 4,000 to 5,500 W generator: enough for the refrigerator, freezer, sump pump, gas furnace blower, lights and electronics. Adding a window AC, microwave and well pump takes you to 7,500 to 9,500 W. Running the whole house, including central air and an electric water heater, needs about 12,840 running watts, which means a home standby generator.
The right size depends on what has to run at the same time, not on the size of your house. Here are three typical setups, calculated with the same values as our generator size calculator.
Three typical setups
| Setup | Running watts | Starting watts | Generator to shop for |
|---|---|---|---|
| Essentials Fridge, freezer, sump pump, gas furnace blower, lights, Wi-Fi, TV, laptop | 2,340 W | 3,890 W | 4,000 to 5,500 W portable |
| Essentials plus comfort Everything above, plus a window AC, microwave and a 1/2 HP well pump | 5,740 W | 7,290 W | 7,500 to 9,500 W portable |
| Whole house Essentials plus microwave, well pump, 3-ton central AC and an electric water heater | 12,840 W | 19,340 W | Home standby |
The generator class adds 20% headroom to the running watts and checks that the generator can handle the startup surge. Starting watts assume only the biggest motor starts at any one moment. To use your own appliances, open the generator size calculator.
Why starting watts matter so much
Motors in fridges, pumps, furnace blowers and air conditioners need a short burst of extra power to start. A central air conditioner is the extreme case: it can pull several times its running watts for a second. That single surge, not your total running load, is often what decides the generator size.
You can keep surges down by starting big loads one at a time, and by having an HVAC technician install a soft-start kit on the air conditioner.
Portable or standby?
For essentials, a portable generator in the 4,000 to 9,500 W range is the most common choice, connected through a transfer switch or interlock. A home standby generator costs far more but starts automatically, runs on natural gas or propane, and can cover the whole house, including central air.
240 V appliances need the right outlet
Well pumps, central AC, electric water heaters and dryers run on 240 V. Your generator needs a 240 V outlet (usually L14-30 or 14-50), and the connection to your panel must go through a transfer switch or interlock installed by a licensed electrician.
Generator safety
Carbon monoxide from portable generators kills people every storm season. The U.S. Consumer Product Safety Commission advises running a generator outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents. Never run one in a garage, basement or shed. Install battery-powered CO alarms inside.
Frequently asked questions
What size generator do I need to run a whole house?
A whole house with central air and electric water heating needs roughly 12,840 running watts and close to 19,340 W at startup, which in practice means a home standby generator sized by an electrician. Without those two loads, most homes manage with a 4,000 to 9,500 W portable generator.
Will a 5,000 watt generator run a house?
It will run the essentials in most homes: refrigerator, freezer, sump pump, gas furnace blower, lights and electronics. It will not run central air, an electric water heater, an electric range or an electric dryer.
Will a 7,500 watt generator run central air?
Usually not. A 3-ton central AC can need 10,000 W or more to start. A soft-start kit installed by an HVAC technician can reduce that surge enough for some large portable generators.
How do I connect a generator to my house?
Through a transfer switch or an interlock kit installed by a licensed electrician, with a generator inlet box outside. Never backfeed the house through a wall outlet.
Sources and method
- U.S. Consumer Product Safety Commission: carbon monoxide deaths and portable generator safety
- ENERGY STAR (U.S. EPA): certified appliance energy data
Runtime figures assume 85% of battery capacity reaches AC appliances and typical US appliance ratings. See exactly how we calculate. Found a mistake? Tell us and we will fix it.