Generator Sizing Calculator
Size a standby generator from running and motor starting loads.
About Generator Sizing Calculator
Motor starting current, not total running load, usually sets generator size. Only the largest motor is assumed to start at once, since simultaneous starts are unlikely and would be prohibitively expensive to design for. A generator that carries the running load but cannot start the well pump or air conditioner is functionally useless during an outage, which is why the surge figure matters most.
Formula
Generator Sizing
Governing load = max(running load, running load − motor + motor × starting multiplier)
- starting multiplier
- Induction motors draw around three times running current on start
- derate
- Generators lose output at altitude and high ambient temperature
Worked example
With the default values below, Generator Sizing Calculator returns Generator Size Required of 9.474. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total running loadminimum 0 W
- 6,000 W
- Largest motor running wattsminimum 0 W
- 1,500 W
- Motor starting multiplieraccepts 1–8 x
- 3 x
- Power factoraccepts 0.4–1
- 0.8
- Altitude and temperature derateaccepts 0–40 %
- 5 %
Results
- Generator Size Required
- 9.474 kW
- Peak Starting Load
- 9,000 W
- Generator Size
- 11.842 kVA
- Surge Headroom Above Running Load
- 3,000 W
- Derated Output at Site Conditions
- 9,000 W
Step by step
- 1Values used
- 2Generator Sizing
- 3Generator Size Required= 9.474 kW
- 4Peak Starting Load= 9,000 W
- 5Generator Size= 11.842 kVA
- 6Surge Headroom Above Running Load= 3,000 W
- 7Derated Output at Site Conditions= 9,000 W
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Generator Sizing calculated?
Governing load = max(running load, running load − motor + motor × starting multiplier) Motor starting current, not total running load, usually sets generator size. Only the largest motor is assumed to start at once, since simultaneous starts are unlikely and would be prohibitively expensive to design for.
Why does Generator Sizing matter?
A generator that carries the running load but cannot start the well pump or air conditioner is functionally useless during an outage, which is why the surge figure matters most.
What values do I need to enter?
This calculator takes 5 inputs: Total running load, Largest motor running watts, Motor starting multiplier, Power factor, Altitude and temperature derate. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.