How to Choose the Right Diesel Generator Size for Your Power Needs
Estimated reading time: 6–7 minutes
When it comes to selecting a diesel generator for industrial, commercial, or backup power use, size matters—critically. A generator that's too small may fail under heavy loads or burn out prematurely. One that's too large may operate inefficiently, leading to higher fuel and maintenance costs. This guide helps you understand how to accurately size a diesel generator for your specific needs.
1. Understand Your Power Requirements
Start by calculating your total electrical load in kilowatts (kW). This includes:
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Running (continuous) loads – equipment that operates steadily
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Starting (peak) loads – motors or compressors that draw extra power at startup
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Non-linear loads – devices like UPSs or variable frequency drives that cause power distortion
✅ Formula:
Total Required Generator Size (kW) = (Running Load kW + Starting Load kW) ÷ Power Factor
Note: Use a conservative power factor of 0.8 for most industrial applications.
2. Know the Type of Loads You’ll Be Powering
| Load Type | Description | Generator Sizing Impact |
|---|---|---|
| Resistive | Heaters, lights | Easy to calculate, no surge |
| Inductive | Motors, compressors, elevators | High startup current—requires oversizing |
| Capacitive | LED drivers, certain electronics | May cause harmonics—need AVR and THD control |
| Non-linear | Computers, VFDs, data servers | Needs generator with good voltage regulation and low THD |
3. Consider Your Application Environment
| Sector/Use Case | Recommended Load Factor | Typical Generator Range |
|---|---|---|
| Construction Sites | 70–90% | 60–500 kVA |
| Hotels & Resorts | 60–80% | 100–1000 kVA |
| Farms/Greenhouses | 50–70% | 40–250 kVA |
| Factories & Workshops | 70–90% | 100–2000 kVA |
| Data Centers | 80–95% | 500–3000 kVA (N+1 redundant) |
If you’re powering critical infrastructure or medical equipment, consider using two or more generators in parallel for redundancy.
4. Account for Startup Surges
Many electric motors draw 3 to 7 times their rated power during startup. For example:
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A 15 kW motor could require up to 105 kW for 1–2 seconds
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Consider adding Automatic Voltage Regulators (AVR) and Electronic Governors to handle surges smoothly
You can also use soft starters or VFDs to minimize inrush current and reduce the generator size needed.
5. Include Future Expansion
Always add 10–20% headroom for future machinery, building extensions, or seasonal load growth. Oversizing slightly is better than running the generator at 100% load continuously.
6. Single vs. Multiple Generator Setup
Sometimes it's better to install two smaller units in parallel than one large generator. Benefits include:
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Redundancy (N+1 setup)
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Lower fuel use during partial load periods
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Easier maintenance with one unit offline
7. Tools You Can Use
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Load Audit Sheets (downloadable from our site)
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Generator Sizing Calculators (contact us for access)
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Free Engineering Consultation (we offer sizing support for global clients)
8. Mistakes to Avoid
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❌ Choosing a generator based only on kVA, without verifying real kW load
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❌ Ignoring peak loads, which can cause frequent shutdowns
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❌ Buying used units without matching voltage and frequency specs (e.g., 50 Hz vs 60 Hz)
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❌ Underestimating altitude or ambient temperature, which reduces effective capacity
Example: Sizing for a Medium-Size Hotel (180 Rooms)
| Load | Quantity | Unit Power (kW) | Total (kW) |
|---|---|---|---|
| HVAC Compressors | 3 | 15 | 45 |
| Kitchen Equipment | – | – | 25 |
| Lighting | – | – | 20 |
| Elevator Motors | 2 | 10 | 20 |
| Laundry Machines | 4 | 5 | 20 |
| Total Running Load | – | – | 130 kW |
| Add Surge Margin (30%) | – | – | 170 kW total |
| Recommended Generator: 200–220 kVA (3-phase, 400V, 50 Hz) |
Conclusion: Match Load to Capacity, Not Guesswork
Sizing your diesel generator is both an art and a science. It requires understanding not just total load, but load behavior, surge factors, and future expansion. An accurately sized genset saves you fuel, reduces wear, and guarantees power reliability.
Author: Technical Support Team – [Ningde City Lion Motor Co., Ltd.]
Last Updated: June 2, 2025