How to Size a Diesel Generator: Complete Guide for Industrial Buyers

What Size Diesel Generator Do I Need?

Choosing the wrong size diesel generator is expensive. Undersize it and the unit trips every time a motor starts. Oversize it and you burn extra fuel daily, risk wet stacking, and void your warranty. This guide walks through the exact sizing process our engineers use for industrial buyers.

Step 1: Calculate Your Total Load (kW to kVA)

List every piece of equipment the generator will power. For each item, note the running watts (found on the nameplate). Add them all up to get total kW.

Then convert to kVA using the standard industrial power factor of 0.8:

kVA = Total kW ÷ 0.8

Example: If your total load is 80 kW, you need at minimum 100 kVA (80 ÷ 0.8 = 100).

Step 2: Account for Motor Starting Current

This is where most sizing mistakes happen. Electric motors draw 4 to 7 times their running current during startup. If you size only for running load, the generator will sag or stall when the largest motor kicks in.

Rule of thumb: Add 25-30% to your calculated kVA if you have significant motor loads (pumps, compressors, HVAC). For the 100 kVA example above with motors, consider a 125 kVA unit.

Step 3: Standby vs Prime Power – Which Rating?

Diesel generators come with different duty ratings:

• Standby (ESP): Emergency backup during utility outages. Sized at 100% of peak load. Most common for commercial buildings.
• Prime (PRP): Primary power source where no grid exists, or for peak shaving. Size so your load is 70-80% of the prime rating.
• Continuous (COP): Non-stop operation. Size so load is ~70% of rating for optimal lifespan.

If your facility has frequent power cuts, you need a prime-rated unit. For backup-only, standby is sufficient and cheaper.

Step 4: Silent vs Open Type

Open-frame generators produce 90-110 dB. Silent canopy models reduce this to 65-75 dB. If the unit sits near offices, hospitals, or residential areas, go silent. Many municipalities cap noise at 65-70 dBA at the property boundary.

Step 5: Plan for Future Growth

Most facilities expand within 2-3 years. Leave 20-25% capacity headroom. It is cheaper to buy slightly larger now than to replace the entire unit later.

Recommended Diesel Generator Sizes by Application

• Small office / retail: 20-50 kVA
• Mid-size warehouse: 100-250 kVA
• Construction site: 30-150 kVA
• Data center: 500 kVA and above (N+1 redundancy recommended)
• Hospital: 200-1000 kVA depending on facility size
• Industrial plant: 250 kVA and above

Common Sizing Mistakes to Avoid

Oversizing: Running below 30% load causes wet stacking – unburned fuel builds up in the exhaust, cutting engine life by 30-50%.

Undersizing: Voltage dips during motor starts can damage sensitive electronics and trip breakers.

Ignoring altitude and temperature: Generators derate at high altitude and extreme heat. At 1,000m elevation, expect ~10% power loss.

Need Help Sizing Your Generator?

Send us your equipment list and our engineers will recommend the right size free of charge. As a diesel generator manufacturer in China since 2014, PAVO supplies 9-3750 kVA gensets to over 30 countries.