The right generator size depends on your facility's total electrical load, including motor starting surges, which can be 6–8 times the running load. A 100 kW unit covers most small commercial buildings; mid-size industrial facilities typically need 200–500 kW; hospitals, data centres, and heavy manufacturing require 500–2,000+ kW.
• Undersized generators overload and fail. Oversized generators waste fuel and cause wet stacking.
• Motor starting surges draw 6–8× running current — your generator must handle the peak, not just the steady state.
• Most industrial facilities above 30 kW need 3-phase power.
• A professional load analysis is the only way to size accurately. Rule-of-thumb charts are starting points, not specifications.
Why Generator Sizing Matters
An undersized generator will overload under peak demand, triggering protective shutdowns — or worse, damaging the alternator windings. The consequences are immediate: lost production, spoiled inventory, safety system failures, and potential equipment damage across your facility.
An oversized generator creates a different set of problems. Running consistently below 30% load causes wet stacking — unburned fuel accumulates in the exhaust system, cylinder walls, and turbocharger. Over time, this damages injectors, fouls exhaust valves, and can void your warranty. You are also paying more upfront for capacity you will never use, and burning more fuel than necessary at partial load.
The target operating range is 50–75% of rated capacity under normal conditions, with headroom for surge loads. This is where diesel engines run most efficiently and maintain the longest service life.
How to Calculate Your Generator Load
A proper sizing assessment follows three steps:
1. List all connected loads. Every piece of electrical equipment that the generator must power — lights, HVAC, motors, pumps, IT systems, production equipment, life safety systems. Record each item's rated wattage or amperage.
2. Separate essential from non-essential. Not everything needs backup power. Essential loads typically include life safety systems, refrigeration, security, communications, and critical production equipment. Non-essential loads (decorative lighting, non-critical HVAC zones, break room appliances) can be shed to reduce the required generator size.
3. Account for motor starting surge. This is where most sizing errors occur. Large induction motors — HVAC compressors, elevator drives, pump motors, conveyor systems — draw 6–8 times their running current during startup. A 50 HP motor running at 37 kW may pull 220–300 kW for 5–10 seconds on startup. Your generator must handle this transient load without voltage or frequency dip exceeding acceptable limits (typically <10% voltage dip).
Sequenced starting (staggering motor startups) can reduce the peak surge requirement, allowing a smaller generator. Variable frequency drives (VFDs) on large motors eliminate surge entirely but add cost.
Generator Size by Facility Type
| Power Range | Typical Application | Examples |
|---|---|---|
| 15–50 kW | Small commercial, telecom | Cell towers, ATMs, small offices, food trucks, residential backup |
| 50–150 kW | Mid-size commercial, light industrial | Grocery stores, warehouses, construction trailers, small factories, restaurants |
| 150–500 kW | Large commercial, industrial | Hotels, mid-size manufacturing, water treatment, retail distribution centres, schools |
| 500–1,000 kW | Heavy industrial, critical infrastructure | Mining camps, large manufacturing, Tier III data centres, regional hospitals |
| 1,000–2,000+ kW | Utility-scale, mission-critical | Tier IV data centres, large hospital complexes, oil & gas, parallel power plants |
These ranges are guidelines. A 10,000 sq ft warehouse with minimal HVAC may need only 75 kW, while the same footprint running CNC machines and welding equipment could require 300+ kW. The load determines the size — not the building.
Single-Phase vs 3-Phase Power
Single-phase generators produce one AC waveform and serve residential and light commercial loads under 25 kW. They power lighting, small appliances, and single-phase motors.
Three-phase generators produce three waveforms offset by 120 degrees, delivering power more efficiently for industrial equipment. You need 3-phase power for:
- Large motors above 10 HP (HVAC, pumps, conveyors, elevators)
- Commercial HVAC systems
- Welding equipment
- CNC machines and industrial production equipment
- Most electrical panels above 200A
Nearly all industrial generators above 30 kW are 3-phase. If your facility's main panel is 3-phase — and it almost certainly is if you operate industrial equipment — your generator must match.
Common Sizing Mistakes
Ignoring motor starting surge. The most frequent error. A facility with 200 kW running load but three large motors that start sequentially may need a 350–400 kW generator to handle the transient peaks.
Sizing for 100% load. Running a generator at full rated capacity continuously shortens engine life and leaves no headroom for unexpected loads. Target 50–75% utilisation under normal operation.
Confusing standby and prime ratings. A 500 kW standby-rated generator is not the same as a 500 kW prime-rated unit. Standby ratings allow higher output for limited hours; prime ratings are derated for continuous operation. Using a standby-rated generator for prime duty will void the warranty.
Forgetting future expansion. If you plan to add production equipment, expand your facility, or increase HVAC capacity within 3–5 years, factor that load into your sizing now. Adding a larger generator later is far more expensive than sizing correctly upfront.
Not accounting for altitude and temperature. Diesel engines lose approximately 3% of rated output per 300 metres above sea level and 2% per 5.5°C above 25°C ambient. A generator rated at 500 kW at sea level and 25°C may deliver only 450 kW at 1,500 metres and 40°C.
Get the Right Generator for Your Facility
Mulciber supplies diesel generators from 15 kW to 2,000 kW paired with Perkins, Volvo, John Deere, Cummins, and MTU engines. Every unit is configured for your specific application — standby, prime, or continuous duty.
Read our complete diesel generator guide for detailed information on fuel types, engine brands, and purchasing considerations. Check our FAQ page for answers to common generator questions, or explore generator rental options for temporary power needs.
Need help sizing? Contact our team for a free load assessment and sizing recommendation. We respond within one business day with a formal quote and full technical specifications.