About the Generator Sizing Calculator
What is the Generator Sizing Calculator?
An undersized generator trips when motors start; an oversized one wastes fuel. This generator size calculator checks both the continuous running load and the surge when the largest motor starts.
It then picks the next standard DG set rating used in India, such as 62.5, 82.5 or 125 kVA.
How to use the Generator Sizing Calculator
- Total Running Load (kW) (sample: 40)
- Load Power Factor (sample: 0.8)
- Largest Motor Started on the DG (kW, 0 if none) (sample: 7.5)
- That Motor's Starting Method (sample: Star-delta (≈2.5×))
- Future Expansion Margin (%) (sample: 20)
- Press Size the Generator (results also refresh as you type). The result card shows the Recommended DG Set, along with Minimum rating needed, Running load, Needed for continuous running (80% loading + margin) and Needed for motor starting.
Formula used
kVA = max(Running kVA × (1 + margin) ÷ 0.8, (Other running kVA + Motor starting kVA) ÷ 1.5)
- Running kVA = kW ÷ power factor
- Motor starting kVA = motor running kVA × 6 (DOL), 2.5 (star-delta), 3 (soft starter) or 1.5 (VFD)
Assumes the set runs at no more than 80% continuous load and tolerates about 150% briefly during motor starting.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Total Running Load (kW) | 40 |
| Load Power Factor | 0.8 |
| Largest Motor Started on the DG (kW, 0 if none) | 7.5 |
| That Motor's Starting Method | Star-delta (≈2.5×) |
| Future Expansion Margin (%) | 20 |
| Result | Value |
|---|---|
| Recommended DG Set | 82.5 kVA |
| Minimum rating needed | 75 kVA |
| Running load | 50 kVA |
| Needed for continuous running (80% loading + margin) | 75 kVA |
| Needed for motor starting | 43.1 kVA |
| Governs | Running load |
Tips for using the Generator Sizing Calculator
- Start the largest motor first, when the set is lightly loaded.
- Soft starters and VFDs can cut the required DG size substantially.
- CPCB IV+ emission norms apply to new DG sets up to 800 kW.