Generator Size Calculator
Watts needed to run your home.
Enter your values
ⓘ Live results
Results
- Minimum generator size
- 8,000 W
- Running watts
- 4,200 W
- Peak demand
- 6,400 W
- Fuel needed
- 9 gallons
- In kilowatts
- 7.68
Quick answer
Size a generator on total running watts plus the largest single starting surge, then add 20% headroom.
How to use the Generator Size Calculator
- 1Add the running watts of everything you need.
- 2Find the largest single starting surge (usually a fridge or pump).
- 3Add 20% headroom.
Worked example
Using the values the calculator loads with:
Inputs
- Total running watts4200 W
- Largest starting surge2200 W
- Runtime needed12 hours
- Fuel use0.75 gal/hour
Results
- Minimum generator size8,000 W
- Running watts4,200 W
- Peak demand6,400 W
- Fuel needed9 gallons
- In kilowatts7.68
Frequently asked questions
What are typical loads?
Fridge 700 W running, sump pump 800 W, window AC 1200 W, furnace fan 800 W.
Do I need a transfer switch?
Yes for hardwired circuits — never backfeed through an outlet.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.