HesapHanem

Generator Sizing Calculator

Enter your total running watts, the largest motor's starting surge and a safety margin to size your generator.

How it is calculated

Enter the total running watts of everything that will be on together, the extra surge of the single largest motor (its starting watts minus its own running watts), and a safety margin in percent. With m = 1 + reserve / 100, the tool computes the peak demand (running + surge) × m and the continuous load running × m. The recommended generator is the peak figure in kW, because the generator must survive the worst-case start while everything else is already running, and the continuous figure shows the steady load it must carry. The tool assumes only one motor starts at a time and does not correct for altitude, temperature, fuel type or power factor. Reserve may be 0 to 100 %.

Worked example

Running loads total 3,200 W. The largest motor needs an extra 1,800 W when it starts, and you want a 20 % margin, so m = 1.20. Peak demand = (3,200 + 1,800) × 1.20 = 6,000 W, so the recommended generator is 6.00 kW. The continuous load is 3,200 × 1.20 = 3,840 W, or 3.84 kW. A generator rated below 6 kW could stall when that motor starts, even though its rating exceeds the running total.

Try this example →

Common mistakes

  • Entering the motor's full starting watts as the surge: the tool adds the surge on top of the running total, so enter only the extra (starting minus running watts), or the motor is counted twice.
  • Leaving out the running watts of the largest motor itself from the running total, which understates the load.
  • Adding the surges of every motor: only the largest starts on top of loads already running, if you stagger startups.
  • Entering amps or volt-amps as watts without converting, which skews the whole result.
  • Choosing a generator right at the recommended figure for use at high altitude or high heat, where output drops and a larger margin is needed.

About this calculator

Sizing a generator correctly requires more than adding up the wattage of everything you plan to run, because motors — pumps, compressors, air conditioners, refrigerators — draw a brief starting (surge) current that can be 2–3 times their running wattage when they switch on. The generator must be sized for total running watts + (the largest motor's starting surge − its own running watts) + a safety margin, since only one motor typically starts at a time while the others are already running.

Homeowners buying a backup or portable generator, and RV or job-site users, run this calculation before purchase because undersizing causes the generator to stall or trip its breaker the moment a compressor kicks on, even if the running-watts total looked comfortably within the generator's rated capacity — the starting surge, not the steady load, is usually what determines the minimum generator size.

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