Standby & Emergency Generator Sizing Calculator
Two sizing approaches for an emergency or standby generator: a fast connected-load-plus-continuous-margin estimate for early planning, and a running-kVA-plus-largest-motor-starting-kVA method for when motor starting data is available.
What it calculates
- Planning-grade generator size from connected load with a continuous-duty margin
- Running kVA plus the starting kVA of the largest connected motor, for a professionally-grounded estimate
- Next standard generator size (10 through 150 kW)
Typical inputs
- Connected load, or running kVA of all loads
- Largest motor's locked-rotor / starting kVA, if using the motor-starting method
Known limits
- NEC 700.4/701.4 require the alternate source to have "adequate capacity and rating" for the load — the Code itself gives no numbered formula. Neither method here substitutes for the generator manufacturer's full transient/voltage-dip study.
Frequently asked
Why does the NEC not specify an exact generator sizing formula?
700.4 and 701.4 deliberately leave sizing to engineering judgment ('adequate capacity and rating'), because the right size depends on motor starting behavior, voltage-dip tolerance and load diversity specific to the installation — factors a single Code formula can't capture. A manufacturer's transient study is the professionally correct final check.
Autodesk and Revit are registered trademarks of Autodesk, Inc. SYMATEQ is not affiliated with, endorsed by, or sponsored by Autodesk, Inc. SYMATEQ Electrical Calculators is an independent add-in for Autodesk® Revit®. Calculations are engineering aids and do not replace review by a licensed professional engineer or confirmation with the authority having jurisdiction.