Generator Size Calculator - How Many Watts Do You Need for Your Home or Job Site?
Generator Size Calculator
Add up running watts and the biggest starting surge
Running and Starting Watts
Appliances with motors draw a short surge when they start. The calculator counts every running load plus the single biggest surge.
Headroom
A generator should not run at its limit. Add a margin, 20 percent by default, so it runs cooler and lasts longer.
A Standard Size
The result is matched to common generator sizes so you know what to shop for.
Private
Your list is not stored.
How to Size a Generator
Add up the running watts of everything you want to power at the same time. Then find the appliance with the biggest difference between its starting and running watts, usually a motor such as a refrigerator, sump pump or air conditioner, and add that extra surge once. Motors do not all start at the same instant, so you do not need to add every surge. The total is the least a generator must supply, and a margin of 10 to 25 percent keeps it away from its limit.
Generators are rated by running watts, and a smaller number, the starting or surge rating, for a few seconds. Ratings on appliance labels are sometimes in amps: multiply amps by volts to get watts. Some loads, such as electric ovens, dryers and water heaters, use thousands of watts and are best left off a small generator. A generator connected to household wiring needs a proper transfer switch installed by a licensed electrician, to protect you and utility workers from back-feed.
Key Takeaways
- Surge counted once: Only the single largest starting surge is added, because motors rarely start at exactly the same moment.
- Check the labels: Real wattage depends on the model. The numbers in the example are typical, not exact.
- Related calculators: See also the Electricity Calculator and the Wire Size Calculator.