Generator Size Calculator
Size a generator correctly by adding your running watts to the biggest motor startup surge.
Result
How to use
- Enter your values in the fields above.
- Press Calculate to see your result instantly.
- Use the Share button to copy a link to your result.
About this calculator
Sizing a generator correctly means adding two different kinds of load: the running watts of everything you plan to power at once, plus the single largest starting (surge) watts spike on top of that — since motors in refrigerators, well pumps, air conditioners and power tools briefly draw two to three times their running wattage the instant they switch on. Only the largest motor's surge needs to be added, because appliances rarely all start simultaneously.
Electricians, generator dealers and homeowners planning for storm outages use this running-plus-surge method to avoid two common mistakes: undersizing a generator that stalls the moment a well pump or AC compressor kicks on, or oversizing one that wastes fuel running mostly idle. Contractors sizing portable generators for jobsite power tools apply the same logic to the largest tool's startup draw.
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