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.