Generator Sizing Calculator Formula

Understand the math behind the generator sizing calculator. Each variable explained with a worked example.

Use the Generator Sizing Calculator

Formulas Used

Running Load

running_kw = running_watts / 1000

Starting Surge

surge_kw = starting_surge / 1000

Minimum Generator Size

generator_kw = (running_watts + starting_surge) / 1000

Variables

VariableDescriptionDefault
running_wattsTotal Running Watts(W)5000
largest_motor_wattsLargest Motor Running Watts(W)2000
starting_multiplierMotor Starting Multiplier(x)3
starting_surgeDerived value= largest_motor_watts * starting_multiplier - largest_motor_wattscalculated

How It Works

Generator Sizing

Minimum kW = Running Watts + Starting Surge

Motors (AC units, pumps, refrigerators) draw 2 to 6 times their running watts during startup. Add the surplus starting watts of the largest motor to the total running watts of everything else.

Worked Example

5,000W running load, largest motor is 2,000W with 3x starting surge.

running_watts = 5000largest_motor_watts = 2000starting_multiplier = 3
  1. 01Running = 5,000W
  2. 02Surge = 2,000 x 3 - 2,000 = 4,000W extra
  3. 03Total = 5,000 + 4,000 = 9,000W = 9.0 kW
  4. 04Select a 10 kW generator.

Ready to run the numbers?

Open Generator Sizing Calculator