Generator Sizing Calculator
Minimum generator size for a set of household or site loads.
Enter your values
Enter the total running watts in W.
Enter the largest single surge watts in W.
Enter the that load's own running watts in W.
Enter the safety margin in %.
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
- 7,150 W
- Minimum generator size
- 7.15 kW
- Total running load
- 3,500 W
Quick answer
Generator size should cover total running watts plus the largest single starting surge.
Size >= running watts + largest surge - that load's running watts
At a glance
| What it does | Minimum generator size for a set of household or site loads. |
|---|---|
| Category | Engineering |
| Inputs needed | Total running watts, Largest single surge watts, That load's own running watts, Safety margin |
| Main output | Minimum generator size |
| Formula | Size >= running watts + largest surge - that load's running watts |
| Cost | Free — no sign-up, no download |
How to use the Generator Sizing Calculator
- 1Add up the running wattage of every load you'll run at once.
- 2Identify the single load with the highest starting surge (often a well pump, AC unit or fridge compressor).
- 3The generator must cover the running total plus that load's surge above its own running watts.
Inputs explained
- Total running watts(W)
- Enter the total running watts in W.
- Largest single surge watts(W)
- Enter the largest single surge watts in W.
- That load's own running watts(W)
- Enter the that load's own running watts in W.
- Safety margin(%)
- Enter the safety margin in %.
Worked example
Using the values the calculator loads with:
Inputs
- Total running watts3500 W
- Largest single surge watts4500 W
- That load's own running watts1500 W
- Safety margin10 %
Results
- Minimum generator size7,150 W
- Minimum generator size7.15 kW
- Total running load3,500 W
Frequently asked questions
Why not just add all surge wattages together?
Surges are brief and rarely overlap; only the single largest surge typically needs headroom above the running total.
Should I size for future loads?
Yes, add margin for anything you might plug in later to avoid buying a second generator.
What do I need to enter into the Generator Sizing Calculator?
Just 4 values: total running watts, largest single surge watts, that load's own running watts, safety margin. 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 Sizing Calculator work out the answer?
Generator size should cover total running watts plus the largest single starting surge. It applies the formula Size >= running watts + largest surge - that load's running watts and shows the working so you can check each step by hand.
Is the Generator Sizing 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.