Generator Sizing Calculator (kVA)
Size a diesel generator for a telecom site or building load.
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Generator Sizing Calculator (kVA) by TheCalcTools
How the Generator Sizing Calculator (kVA) works
Enter the total running load, its power factor and the largest motor or compressor that will start.
Running kVA = load ÷ PF with a growth margin; starting kVA includes the motor’s starting multiple.
Derate for altitude above 1,000 m (about 1% per 100 m) and ambient above 40°C (about 1% per °C), then pick the next standard size.
The formula
Worked example
Using the default values shown in the calculator:
| Total running load (kW) | 18 |
|---|---|
| Load power factor | 0.85 |
| Largest motor/compressor (kW) | 5.5 |
| Starting multiple of motor | 3 |
| Site altitude (m) | 500 |
| Maximum ambient (°C) | 45 |
| Growth margin (%) | 20 % |
- Running kVA = 18 × 1.2 ÷ 0.85 = 25.4; peak at start = 34.1 kVA.
- Derating factor 0.950, so required = 28.7 kVA; next standard size 30 kVA.
Result:
| Running kVA with margin | 25.4 |
|---|---|
| Peak kVA during motor start | 34.1 |
| Derating factor | 0.950 |
| Required generator (kVA) | 28.7 |
| Next standard size (kVA) | 30 |
Frequently asked questions
Why 0.8 × starting kVA?
A generator can briefly supply more than its continuous rating during a motor start. Checking against about 80% of the peak is a common rule of thumb. Confirm with the generator’s step-load data.
Is this a complete design?
No. A qualified engineer must confirm voltage dip, harmonics, fuel and site-specific limits.
Sources and further reading
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This calculator gives estimates for planning only. Electrical installations must be designed, checked and carried out by qualified personnel following local codes and the equipment manufacturer's data. Never rely on this result alone for safety-critical decisions. See our disclaimer.