Generator Sizing Calculator
Minimum generator size for a set of household or site loads.
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ⓘ Live results
Results
- 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
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.
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.
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.