Generator Sizing Guide: How to Calculate the Right kW

A correctly sized generator runs efficiently, meets voltage and frequency under load, and lasts. A wrong size either stalls on startup or wastes fuel and capital. This guide shows the calculation we use for real installations.

Step 1: List Every Load

Write down every device the generator must supply, with its running power in watts or kW. Include lighting, HVAC, pumps, compressors, servers and any motor-driven equipment.

Step 2: Add Motor Starting Surge

Electric motors draw a large inrush current at startup — often 3x to 7x their running load for a few seconds. A 2 kW compressor may need 10 kW to start. Size for the largest simultaneous starting surge, not just the steady sum.

Step 3: Apply Diversity Factor

Not every load runs at once. A diversity (or coincidence) factor of 0.6–0.8 is common for buildings. Multiply the total by this factor to avoid gross oversizing — but never apply it to critical simultaneous loads.

Step 4: Convert kW to kVA

Generators are often rated in kVA. Use: kVA = kW ÷ power factor (typically 0.8). A 100 kW load at 0.8 pf needs 125 kVA.

Step 5: Add Headroom

Specify roughly 20–25% spare capacity above the calculated peak. This covers future expansion, derating at altitude/high temperature, and ageing.

Common Sizing Mistakes

• Undersizing: the unit trips on starting surge or sags voltage, damaging connected equipment.
• Oversizing: wet stacking in diesel sets, poor efficiency, higher cost.

Single-Phase vs Three-Phase

Small sites (homes, shops) use single-phase 220–240V. Industrial sites use three-phase 380–415V. Match the generator to your distribution.

Need a precise load study? Our team can size from your panel schedule — see case studies or request a quote. You can also browse pre-sized ranges by power band, e.g. 500–1000 kW diesel generators.