What Size Generator Do I Need?
November 9, 2026
To size a generator, add the running watts of everything you'll power at once, then add the extra starting watts of the single largest motor. Choose a generator rated above that total with about 20% to spare. Check the label on each appliance or tool, since real wattages vary.
Running watts vs starting watts
- Running watts keep a device going once it's on.
- Starting watts are the short surge a motor needs to start, often two to three times its running watts. Refrigerators, pumps, compressors and saws all have a surge.
You only need to add the biggest single surge, because motors rarely start at the same moment.
Typical wattages
These are common ranges for planning. Use the numbers on your own equipment when you have them.
| Item | Running watts (about) | Starting watts (about) |
|---|---|---|
| Refrigerator | 600 to 800 | 1,500 to 2,200 |
| Sump pump (1/2 hp) | 1,000 | 2,000 |
| Window air conditioner | 1,000 to 1,500 | 1,500 to 2,200 |
| Space heater | 1,500 | 1,500 |
| Microwave | 1,000 | 1,000 |
| Circular saw | 1,200 to 1,500 | 2,000 to 2,500 |
| LED work lights | 50 to 200 | 50 to 200 |
Worked example
A refrigerator (700 running), sump pump (1,000), lights (200) and a microwave (1,000) need 2,900 running watts. The biggest surge is the sump pump, about 1,000 extra. That's 3,900 watts, so a generator of about 5,000 watts gives you room.
Jobsite vs home
- Jobsites: count every tool that will run at once, and add a light tower for night work. Large towable diesel generators suit big crews.
- Homes: never plug a generator into a wall outlet. Use a transfer switch installed by an electrician, and run the generator outdoors, away from windows, because of carbon monoxide.
Where to rent a generator
Generators are among the most searched rentals in big cities: about 460 times a month in Los Angeles and 390 in New York City. Compare suppliers in your city on Jobsite Jockey.
In short: add running watts, add the largest starting surge, then size up about 20%. See the generator rental guide for portable and towable options.