Generator Size Calculator
Use this generator sizing workflow for early load planning. It keeps starting watts and margin visible but does not choose a listed generator or approve transfer equipment.
- Minimum running rating
- W
- Minimum surge rating
- W
- Minimum running rating
- kW
- Margin multiplier
- x
Calculation details
- Estimated kVA basis
- kVA
- Calculation basis
- Selection boundary
Recent results
Formulas
- Margin multiplier = 1 + margin percent / 100
- Running rating watts = running watts x margin multiplier
- Surge rating watts = (running watts + largest starting watts + other starting watts) x margin multiplier
- Required kVA = surge rating watts / power factor / 1000
Use this calculator
Estimate generator running watts, surge watts, and kVA from entered load assumptions.
Read the result
Review the calculated values with their displayed units and assumptions. The result is a planning aid for the stated inputs, not a final equipment or installation decision.
Check before final work
This generator size estimate uses entered load assumptions only. It does not approve generator selection, transfer equipment, fuel, grounding, utility interconnection, or code compliance.
FAQs
Does this choose a generator model?
No. It estimates running and surge requirements from entered loads. Final generator selection needs manufacturer data and installation review.
Why include starting watts?
Motors and compressors can need extra watts during starting. Enter likely simultaneous starting load so the surge estimate is visible.