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Generator Size Calculator

Size a generator correctly by adding your running watts to the biggest motor startup surge.

How it is calculated

Three steps: required = running + starting, then recommended = required × 1.2, then kVA = recommended / 1000 / 0.8. running is the total watts of everything that will run at the same time, and starting is the extra surge watts of the one motor that starts with the biggest jolt, in watts too. The 1.2 factor adds 20% headroom, and the kVA figure assumes a power factor of 0.8.

Whatever you type in the starting field is added in full, so enter only the surge above that motor's running watts, not its running watts again, if you have already counted them under running. A blank or negative starting value is treated as 0. Running watts must be above zero. Watts are shown as whole numbers and kVA with two decimals.

Worked example

A home needs 3,200 W of running load (fridge, lights, TV, small appliances). The well pump has a startup surge of 2,400 W. Required power = 3,200 + 2,400 = 5,600 W. With 20% headroom: 5,600 × 1.2 = 6,720 W recommended. In kVA, at a power factor of 0.8: 6,720 / 1,000 / 0.8 = 8.40 kVA. So look for a generator rated at about 6,720 W or more, or 8.4 kVA.

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Common mistakes

  • Adding the surge of every motor. The method assumes only the largest start at a time; adding all of them gives an oversized generator, while skipping it gives an undersized one.
  • Using the nameplate wattage of an appliance as its running load when it only cycles on and off. Add up what actually runs together.
  • Entering the motor's total starting wattage in the starting field after already counting its running watts. The tool adds both, so the running part is counted twice.
  • Confusing watts with kVA when shopping. The kVA figure here is calculated from an assumed 0.8 power factor, so a generator's kW rating is usually the number to compare with watts.
  • Treating the recommended figure as a minimum for continuous output. It includes 20% headroom, which is a safety margin, not a measured load.

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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  • Editorial team

    The tool adds the surge of just one motor, which is the usual approach when you can start big loads one at a time. If the well pump and the air conditioner could kick in together, put both surges in the starting field. Also note that the kVA figure assumes 0.8 power factor, while many portable units are rated in watts, so compare watts to watts. Which appliance is the hardest to start in your setup?

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