Generator Sizing Calculator Formula
Understand the math behind the generator sizing calculator. Each variable explained with a worked example.
Formulas Used
Running Load
running_kw = running_watts / 1000Starting Surge
surge_kw = starting_surge / 1000Minimum Generator Size
generator_kw = (running_watts + starting_surge) / 1000Variables
| Variable | Description | Default |
|---|---|---|
running_watts | Total Running Watts(W) | 5000 |
largest_motor_watts | Largest Motor Running Watts(W) | 2000 |
starting_multiplier | Motor Starting Multiplier(x) | 3 |
starting_surge | Derived value= largest_motor_watts * starting_multiplier - largest_motor_watts | calculated |
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.
- 01Running = 5,000W
- 02Surge = 2,000 x 3 - 2,000 = 4,000W extra
- 03Total = 5,000 + 4,000 = 9,000W = 9.0 kW
- 04Select a 10 kW generator.
Frequently Asked Questions
What appliances have the highest starting surge?
Central AC units (3-5x), well pumps (3-5x), and sump pumps (2-3x) have the highest starting surges. Resistive loads like heaters and lights have no surge.
Should I size for the whole house?
A whole-house generator typically needs 15-25 kW. For a portable backup, prioritize essential circuits (fridge, sump pump, furnace, lights) and size accordingly.
Ready to run the numbers?
Open Generator Sizing Calculator