Generator size calculator
List what has to run during an outage. You get the running watts, the startup surge, and the size class to shop for.
Size your generator
Add an appliance
How generator sizing works
Generators carry two ratings. Running watts are what the generator can supply continuously. Starting watts are the higher output it can hold for a few seconds while a motor starts.
To size one, add the running watts of everything that will run at the same time. Then take the single appliance with the biggest gap between starting and running watts and add that gap. You only need to cover one motor starting at a time, because in practice they rarely start at the same instant, and you can make sure they don't.
The calculator then adds 20 percent headroom to the running total. Generators run hotter, louder and less efficiently near their limit, and appliance labels are averages.
Choosing the type
| Type | Typical running watts | Best for |
|---|---|---|
| Inverter (portable) | 1,800 to 5,000 | Fridge, lights, electronics, CPAP. Quiet and safe for sensitive electronics. |
| Open-frame portable | 3,500 to 12,000 | Essentials plus pumps and a furnace, usually through a transfer switch. |
| Home standby | 10,000 to 26,000+ | Whole-home backup that starts automatically. Professionally installed. |
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, even with the door open. Install battery-powered CO alarms inside.
When shopping, look for models with an automatic CO shut-off, which turns the engine off if carbon monoxide builds up around it.
240 V appliances
Well pumps, central AC, electric water heaters and electric dryers run on 240 V. A generator needs a 240 V outlet (usually an L14-30 or 14-50) and a transfer switch or interlock to feed them. That connection is a job for a licensed electrician.
Frequently asked questions
What size generator do I need to run a house?
For essentials like a refrigerator, sump pump, furnace blower, lights and electronics, most homes need a 4,000 to 5,500 W generator, or 7,500 to 9,500 W if you add a well pump and window AC. Running central AC, an electric water heater or an electric range usually pushes you into standby generator territory.
What are starting watts?
Starting watts, or surge watts, are the short burst of extra power a motor needs to start. Only the largest motor starting at any one moment has to be added on top of your total running watts.
Is an inverter generator better?
Inverter generators produce cleaner power, run quieter and use less fuel at light loads. They cost more per watt, so they make the most sense below about 5,000 watts.
Can I plug a generator into a wall outlet?
No. Backfeeding a house through an outlet is illegal in most places and can electrocute utility workers. Use extension cords or have an electrician install a transfer switch or interlock kit.
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.