How to Size a Diesel Generator: kVA Calculation with Worked Examples

To size a diesel generator: add up your running load in kW, divide by the power factor (0.8 for mixed loads), then multiply by a 1.20–1.25 safety margin. A site with an 80 kW peak load needs (80 ÷ 0.8) × 1.25 = 125 kVA. If large motors start direct-on-line, verify the set can absorb the starting surge — typically 3–7× a motor's running current.

Prefer to skip the arithmetic? Our kVA calculator does this in 30 seconds.

Step 1 — Total your connected load

List every machine and appliance that will run on the generator with its nameplate kW. If everything never runs at once, use your realistic maximum demand — typically 60–80% of the total connected load.

Step 2 — Convert kW to kVA

Generators are rated in kVA (apparent power); your equipment consumes kW (real power). The bridge is the power factor:

Generator kVA = (Load kW ÷ Power Factor) × Safety Margin

Use 0.8 PF for mixed commercial/industrial loads and a 1.20–1.25 margin for future growth and derating headroom.

Load type	Typical power factor
Heaters, incandescent lighting	1.0
LED lighting, IT equipment	0.90–0.99
Induction motors (full load)	0.80–0.92
HVAC / air conditioning	0.75–0.85
Welding machines	0.50–0.60

Step 3 — Check motor starting surge

An induction motor draws 3–7× its running current at start (DOL starting). Rule of thumb: the generator should handle at least 3× the kW of your largest motor on top of the rest of the running load. Star-delta starters cut the surge to about a third; VFDs eliminate it almost entirely.

Step 4 — Apply site derating

Nameplate ratings assume ~25 °C at low altitude (ISO 8528). Derate roughly 1% per 100 m above 1,000 m altitude and ~2% per 5 °C above 40 °C ambient — real considerations for North Indian summers and hill sites.

Worked examples

Small office: 22 kW demand → (22 ÷ 0.8) × 1.25 = 34 kVA → a 40 kVA set from the Tata-powered RT+ series fits with margin.

Mid-size factory: 85 kW demand including a 15 kW compressor (DOL) → (85 ÷ 0.8) × 1.2 ≈ 128 kVA running, and 3 × 15 = 45 kW starting headroom is covered → a 125 kVA set.

Never chronically undersize or oversize. Running below ~30% load causes wet stacking (carbon build-up); the efficient band is 50–80% load.

Frequently asked

What size generator for a house in India? Most 3–4 BHK homes with 1–2 ACs fall in the 5–15 kVA range; check your largest AC's starting surge.

Is a 20% margin really necessary? Yes — it covers load growth, hot-day derating, and keeps the set in its efficient load band.

Unsure between two ratings? Send us your load list — our sales engineers size it free of charge, or try the kVA calculator.