Generator Size Calculator
Watts needed to run your home.
Enter your values
Enter the total running watts in W.
Enter the largest starting surge in W.
Enter the runtime needed in hours.
Enter the fuel use in gal/hour.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
At a glance
| What it does | Watts needed to run your home. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Total running watts, Largest starting surge, Runtime needed, Fuel use |
| Main output | Minimum generator size |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Total running watts(W)
- Enter the total running watts in W.
- Largest starting surge(W)
- Enter the largest starting surge in W.
- Runtime needed(hours)
- Enter the runtime needed in hours.
- Fuel use(gal/hour)
- Enter the fuel use in gal/hour.
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.
What do I need to enter into the Generator Size Calculator?
Just 4 values: total running watts, largest starting surge, runtime needed, fuel use. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Generator Size Calculator work out the answer?
Size a generator on total running watts plus the largest single starting surge, then add 20% headroom. The working is shown step by step so you can verify the result by hand.
Is the Generator Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.