Pick a generator size for your load. Enter the total running load in kW and, if you have a large motor such as a pump or compressor, its size and how it starts. The calculator checks both the running load and the motor starting surge.
Estimate only, at standard conditions. Altitude, heat, fuel type and harmonic loads reduce output. Ask the generator supplier to confirm for your loads.
Formulas and assumptions
- Running kVA: total kW divided by power factor
- Margin: running kVA times 1.25 for growth and safe loading
- Motor starting: motor kW divided by 0.85, times the starting multiple (6 direct on line, 3 star-delta or soft starter, 2 VFD)
- Starting check: peak kVA divided by 2.5, since a generator can briefly supply about 2.5 times its rating
- kW rating: kVA times 0.8
Example
A 20 kW load at power factor 0.8 is 25 kVA running, or 31.3 kVA with margin. With a 5 HP pump started direct on line the peak is 46.7 kVA, which needs about 18.7 kVA of generator. The running load decides, so a 40 kVA generator (about 32 kW) is the next standard size.
Frequently asked questions
How do I choose a generator size?
Add up the running load in kW, divide by power factor for kVA, add about 25 percent margin, then check that the generator can also start your largest motor.
Why does motor starting matter so much?
A motor can draw about six times its running current when started direct on line. A generator that only fits the running load may stall or drop voltage at that moment.
What is the difference between kVA and kW for a generator?
kVA is the total capacity. kW is the useful part, usually 80 percent of kVA. Always size on both.
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