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Generator Size Calculator

Generator size from running load plus the largest motor start, with altitude derate, the next standard size, and fuel use.

Inputs

Load

Everything you expect to run at the same time, using running watts rather than starting watts.

Running watts of the biggest motor in the load. Well pumps, compressors, and furnaces dominate generator sizing.

Starting watts per running watt. Motors and compressors are typically 3 to 5 times running watts; check the nameplate.

Site and fuel

Generators lose roughly 3.5% of rated output per 1,000 ft above sea level.

From the manufacturer fuel curve at your load. A 7-10 kW portable at half load runs roughly 0.15-0.25 gal/kWh.

Result

Starting load in kW
9.00 kW
Starting load to cover
9,000 W
Nameplate size needed
9.00 kW
Next 2.5 kW standard size
10.0 kW
Output available at your altitude
1.000 x
Fuel at this load
1.00 gal/hr
Runtime per gallon
1.00 hrs

What to check

  • Running watts for everything that will run at once, plus starting watts for the largest motor from its nameplate.
  • Starting multiple from the motor or equipment datasheet: pumps and compressors often start at 3-5 times running watts.
  • Altitude derate: roughly 3.5% of rated output per 1,000 ft above sea level, plus a temperature derate in hot weather.
  • Fuel type, storage, and carbon monoxide placement: never run a portable generator indoors or in an attached garage.
  • Whether a transfer switch and inlet are installed, and what the generator can actually carry as a managed load rather than all at once.

Keep going

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Related guides

Sizing estimate from load and motor figures you enter. It does not evaluate starting waveforms or managed-load transfer switches, and fuel consumption is a rule-of-thumb rather than the manufacturer fuel curve. Confirm with the generator datasheet, and follow the locally adopted National Electrical Code (Article 702 for optional standby systems).

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