Every generator lists two numbers. Running watts (also called rated or continuous watts) is what it can supply for hours. Starting watts (peak or surge watts) is the extra burst it can deliver for a second or two.
Why appliances need starting watts
An electric motor at rest acts almost like a short circuit. For a moment after it switches on, it pulls two to three times its normal power until it spins up. Refrigerators, freezers, sump and well pumps, furnace blowers and air conditioners all do this, and they cycle on and off by themselves all day.
Heaters, light bulbs, toasters and electronics don't have a starting surge: their starting and running watts are the same.
How to size for both
- Add up the running watts of everything that might be on at the same time.
- Find the appliance with the biggest gap between starting and running watts.
- Add that gap to your total. That's your starting peak, the worst moment the generator has to handle.
- Choose a generator whose starting watts cover that peak, and whose running watts are about 20% above your total.
You don't add up every appliance's starting watts, because motors rarely start at exactly the same instant. The calculator does all of this for you.
Why store labels can mislead
Most portable generators are sold by their starting watts. A "5,000-watt" model typically runs about 4,000 watts continuously. Always compare running watts when you shop.