Generator Size Calculator (DG Set kVA)
By Santaji GadepatilinUpdated October 28, 2026
A diesel generator must supply the running load and also survive the surge when the largest motor starts. Undersized sets stall or trip; oversized sets run lightly loaded and wet-stack. This generator size calculator gives the minimum kVA from both checks — see how to calculate generator size for industrial load and how an AMF panel auto-starts a DG.
How to use the generator size calculator
Enter the total running load in kW, its power factor, a margin for growth, the largest motor that starts while the rest of the load is running, and its starting kVA per kW for the starter type. The calculator gives the kVA needed for running and for the motor start; choose a standard rating at or above the larger figure.
Generator size formula
kVArun = kW ÷ PF × (1 + margin)
kVAstart = [(kW − kWmotor) ÷ PF + kWmotor × ks] ÷ 1.5
The 1.5 factor assumes the generator can carry 150% of its rating momentarily during motor starting while keeping voltage dip acceptable. Check the manufacturer’s motor-starting curves for an exact selection.
Example
80 kW running load at 0.8 PF with 20% margin; the last motor is 22 kW on a star-delta starter:
- Running: 80 ÷ 0.8 × 1.2 = 120 kVA
- Starting: (58 ÷ 0.8 + 22 × 2.5) ÷ 1.5 = 85 kVA
- Choose 125 kVA
Starting kVA per kW by starter
| Starter | kVA per kW (approx.) |
|---|---|
| Direct-on-line | 6–7 |
| Star-delta | 2–2.5 |
| Autotransformer | 3–4 |
| Soft starter | 1.5–2.5 |
| VFD | 1.2–1.5 |
Standard DG set ratings (India)
15, 20, 25, 30, 40, 62.5, 82.5, 100, 125, 160, 200, 250, 320, 380, 500, 625, 750, 1,010, 1,250, 1,500 and 2,000 kVA.
Other checks
- Non-linear loads (VFDs, UPSs) may need generator derating or a larger alternator.
- Altitude and temperature reduce output — derate above 1,000 m or 40 °C.
- Step loading: start the largest motors first, when the generator is lightly loaded.
Frequently asked questions
How do I calculate generator size?
Divide the running load in kW by the power factor (usually 0.8) to get kVA, add 20–25% margin, and check that the set can also handle the largest motor starting.
Why use 0.8 power factor?
Generators are rated in kVA at 0.8 PF. A 100 kVA set delivers up to 80 kW.
Why does motor starting matter?
A motor draws several times its running kVA when starting. Most generators can supply about 1.5–2 times their rating briefly while keeping voltage dip around 15–20%; this calculator uses 1.5.
Is a bigger generator always better?
No. Diesel sets run best above 30–40% load; long periods at light load cause wet-stacking and carbon build-up.