Generator Size Calculator: Watts Needed
This generator size calculator answers "what size generator do I need?" for a home, RV or job site. Add the appliances you want to run at the same time, using the preset list or your own running and starting watts. Motors like refrigerators, well pumps and AC units need a surge of extra watts to start, so the calculator adds your total running watts to the single largest starting surge, then adds a safety margin. The result is the minimum running and peak watts to shop for, plus a suggested standard generator size.
Generator Size Calculator
Add your appliances to find the running and starting watts and the generator size you need.
How to use the Generator Size Calculator
- Keep or edit the starter appliance list.
- Add appliances with running and starting watts.
- Set a safety margin, usually 20% to 25%.
- Read the recommended generator size.
Generator sizing
Size = (total running watts + largest single starting surge) × (1 + safety margin)
Features
- Appliance list with presets
- Running and starting watts
- Safety margin
- Suggested standard size
Frequently asked questions
How do I calculate what size generator I need?
Add the running watts of everything you will run at once, then add the largest extra starting watts of any one motor. Add 20% headroom. 3,500 running watts plus a 2,000 W surge is 5,500 W, or about 6,600 W with headroom.
What is the difference between running and starting watts?
Running watts keep an appliance going. Starting (surge) watts are a short burst, often 2 to 3 times higher, that motors need for a few seconds to start.
What size generator runs a whole house?
Essential circuits (fridge, lights, furnace fan, sump pump) often need 5,000 to 7,500 W. Whole-home with central AC can need 15,000 to 22,000 W.
Can a 3,500 watt generator run a refrigerator?
Usually yes. A typical refrigerator runs at about 700 W and needs about 2,200 W to start, leaving room for lights and a few small devices.
Why add a safety margin?
Running a generator at 100% for long periods shortens its life and can trip the breaker. Keeping the load at 80% or less is a common rule.