Generator Sizing Calculator
Size a generator against a real motor starting sequence, the way an engineer would on site. Enter each motor in the order it starts, choose how it is started, and the calculator returns the current it draws to run, the surge it draws to start, and the minimum generator that will carry both.
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Optional. We will send the figures above so you have them on file, and an engineer can pick up the conversation from there.
How the sizing works
A generator is rarely sized by the load it runs. It is sized by the worst instant during start-up: the moment one motor draws its starting surge while every motor before it is already turning. That instant is what this calculator finds.
Each motor is converted to horsepower, then to a running current using an amps-per-horsepower figure that falls as motors get larger. The starting surge is a multiple of that running current, set by how the motor is started — direct on line draws the most, star/delta roughly half, and a soft starter or VSD less again. The calculator walks the sequence you enter, tracks the running total, and reports the largest starting demand it meets along the way.
Two figures come out of that: the size needed to start the load and the size needed to run it. The recommended set is the larger of the two, plus 16% headroom. Ordering matters — starting the largest motor last is the most demanding case, and is usually how a real plant behaves.
These calculations are an estimate only. Site conditions, motor efficiencies and iron losses are not taken into account, and the starting figure assumes an alternator that will accept 200% of rated current momentarily. Altitude, ambient temperature, harmonic distortion from VSDs and non-motor loads all shift the answer. Confirm the final selection with our engineers before specifying.
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