Diesel Generator Sizing Guide: How to Calculate kVA/kW for Your Facility

Why Correct Sizing Matters

An undersized generator will overload, overheat and fail prematurely. An oversized generator runs inefficiently, wastes fuel and increases maintenance costs. Correct sizing ensures reliability and lowest total cost of ownership.

Step 1: List All Loads

Make a list of all equipment that must run during a power outage. Record each item's running power (kW) and starting power (kW or kVA).

Step 2: Apply Diversity Factor

Not all loads run at the same time. Multiply the total running load by a diversity factor, typically 0.6 to 0.8 for commercial buildings and 0.8 to 1.0 for critical facilities like hospitals.

Step 3: Account for Motor Starting Current

Electric motors draw 5-7 times their running current during startup. Use soft starters, VFDs or oversized generators to handle these inrush currents without voltage dip.

Step 4: Convert kW to kVA

Generator capacity is usually rated in kVA. Use the formula: kVA = kW / Power Factor. Typical power factor is 0.8. For example, a 400kW load requires at least 500kVA generator capacity.

Step 5: Add Future Expansion

Add 20-30% spare capacity for future load growth. This also improves fuel efficiency and extends engine life.

Quick Reference Table

Get Professional Sizing Assistance

Jinjia Power engineers can perform load analysis and recommend the right diesel generator size for your project. Send us your load list for a free quotation.