Generator Sizing Calculator
The generator sizing estimator combines total running wattage with additional start-up demand from the largest motor and a planning reserve.
How to use this calculator
- Enter the total running load including the largest motor once, that motor’s running watts, its start-up multiplier and reserve.
- Check the units and definitions before comparing results with another source.
- Change one input at a time when comparing scenarios, and keep the reporting period and assumptions consistent.
Formula used by the generator sizing calculator
Planning watts = (total running watts + motor watts × (start multiplier − 1)) × (1 + reserve%)
- Planning watts = (total running watts + motor watts × (start multiplier − 1)) × (1 + reserve%)
- Starting current, power factor, altitude, generator type and simultaneous load sequences can change requirements. This is a preliminary estimate, not an electrical design.
- Inputs are checked for required values and valid ranges; displayed results are rounded for readability.
Worked examples
Worked example
Total running load 1,800 W, largest motor 600 W, start multiplier 3 and 20% reserve.
Estimated capacity is (1,800 + 600×2)×1.20 = 3,600 W.
Estimated capacity is (1,800 + 600×2)×1.20 = 3,600 W.
Understanding the result
The generator sizing estimator combines total running wattage with additional start-up demand from the largest motor and a planning reserve.
Starting current, power factor, altitude, generator type and simultaneous load sequences can change requirements. This is a preliminary estimate, not an electrical design.
Important notes
Starting current, power factor, altitude, generator type and simultaneous load sequences can change requirements. This is a preliminary estimate, not an electrical design.
The result applies to the numbers and assumptions entered here; verify specifications for safety-critical or regulated uses.
Frequently asked questions
Why is the motor included twice in the formula?
It is not: the motor’s running watts are already in total load; only the extra start-up amount is added.
Can I use this as a final generator specification?
No. Confirm appliance starting requirements and manufacturer ratings with a qualified professional for critical equipment.
How do I use this calculator?
Enter the total running load including the largest motor once, that motor’s running watts, its start-up multiplier and reserve.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.