A diesel generator needs its correct size determination through two steps which require adding all active load running watts together with highest motor starting surge measurement from the largest motor. The total kilowatts (kW) need conversion into kilovolt-amperes (kVA) through division by 0.8 before adding a 20 to 25 percent safety margin.
Rudi used running wattage from his production system to determine his backup generator requirements. The generator started its operation without any issues after the main power supply experienced a failure.
The cooling pumps began their startup sequence. The system voltage experienced a crash because of the inrush current. The system experienced a total shutdown within eight seconds. His organization experienced six hours of downtime which resulted in missing an important delivery deadline.
This story describes a common situation. The most expensive error which occurs during industrial power system design happens when engineers select incorrect equipment sizes. Equipment becomes damaged because undersizing causes overload trips and voltage collapse. Excess capital gets wasted because equipment operates above necessary capacity while engine components fail from carbon accumulation. The organization needs to achieve both results but it can only attain one result.
The company Shandong Huali Electromechanical Co. Ltd. has completed generator set installations which range from 10 kVA to 4,000 kVA across 70 different countries. The guide will present the complete process which our engineering team employs. The procedure helps you determine your load requirements while including motor startup needs and environmental derating factors and it shows you five common errors which lead to maximum expenses for facilities.
Want to gain a comprehensive understanding of diesel generators? (Read A Complete Guide to Diesel Generators) to delve deep into their classification, uses, and selection tips.
Key Takeaways
Calculate running watts plus the largest single starting surge, then convert kW to kVA using a 0.8 power factor.
Add a 20 to 25 percent safety margin for future growth and transient loads.
Apply environmental derating for altitude (3.5 percent per 300 meters) and high temperatures.
Motor starting current, not running load, often decides the final generator size.
Oversizing is as dangerous as undersizing; chronic underloading causes wet stacking and engine damage.
How Do I Calculate What Size Generator I Need?
You measure generator capacity through four distinct phases.
First, inventory every electrical load the generator must power. Second, you need to combine all operational watts with the maximum single equipment starting surge. Third, you need to convert the total kilowatts into kVA by using your established power factor. Fourth, you need to apply a safety margin which ranges between 20 to 25 percent.
The calculation provides the necessary generator capacity which you require. You then select the next standard size up, never down. The calculation shows 312 kVA which requires you to select either a 350 kVA or 400 kVA unit based on the manufacturer options.
The process requires an uncomplicated approach. The majority of industrial purchasers tend to skip essential procedures while making unverified assumptions which result in future expenses. The sections below walk through each stage in detail.
For a visual walkthrough of load calculation and motor starting, manufacturer training videos from Cummins and Atlas Copco provide excellent technical overviews.
Step 1: Inventory All Connected Loads
Start with a complete list. Every motor, pump, compressor, HVAC unit, lighting bank, computer server, and safety system that will run simultaneously must appear on your load sheet.
For each item, record two numbers from the equipment nameplate or technical datasheet:
- Running watts (kW) — the power consumed during normal operation
- Starting watts (surge kW) — the brief power spike needed to start motors or compressors
Running Watts vs. Starting Watts
Most generator failures occur because of electric motor problems. The 30 kW cooling pump requires 150 kW or more for its startup process which lasts between two and five seconds. This phenomenon creates inrush current.
The voltage drop occurs whenyour generator fails to handle the surge. The breakers activate. The motors stop working. The sensitive electronics either reset themselves or experience complete failure.
Different motor types and their starting methods produce different levels of starting surge. Direct-on-line (DOL) starting produces the highest surge. Soft starters and Variable Frequency Drives (VFDs) reduce it significantly. Step 5 presents a comprehensive analysis of different starting methods.
How to Read Equipment Nameplates
Nameplates list power in watts (W), kilowatts (kW), or horsepower (HP). Convert horsepower to kilowatts using 1 HP = 0.746 kW. Some nameplates list current (amps) and voltage instead. Calculate watts using:
Watts = Volts × Amps × Power Factor
For three-phase equipment, use:
Watts = Volts × Amps × Power Factor × 1.732
If the power factor is not listed, assume 0.8 for general industrial loads.
Loads That Are Easy to Forget
Buyers often focus on large machinery and overlook smaller loads that add up. Check your list against these commonly forgotten items:
- Emergency lighting and exit signs
- Fire suppression pumps and alarms
- HVAC systems (compressors have high starting surge)
- Battery chargers and UPS systems
- Security systems and access control
- Exhaust fans and ventilation
- Office equipment and computer servers
- Elevators and conveyor systems
Step 2: Calculate Total Power Requirement
Once your inventory is complete, perform the calculation in three parts.
Adding Running Loads
Add the running kilowatts of every device that operates simultaneously. You should exclude all items which function exclusively on backup power or have scheduled operations that differ from backup times. Your actual operating schedule should be used to calculate your facility’s power requirements, which should not exceed the maximum possible load.
The facility needs to categorize its electrical loads according to different load types. Heaters and incandescent lighting fixtures function as resistive loads which maintain a power factor of 1.0. Inductive loads like motors have power factors between 0.7 and 0.95 depending on loading. Using a single average power factor for your entire facility introduces error.
Adding the Largest Starting Surge
Identify the single largest starting surge on your list. Add only this one surge to your running total. Do not add all starting surges together. Motors rarely start at exactly the same instant.
Total Peak kW = Sum of Running kW + Largest Starting Surge kW
Understanding kW vs. kVA
Generator manufacturers rate their units in kilovolt-amperes (kVA), not kilowatts. The relationship between them is the power factor (PF).
kVA = kW ÷ Power Factor
For most industrial loads, use a power factor of 0.8:
kVA = kW ÷ 0.8
This means a 100 kVA generator delivers approximately 80 kW of real power. If your facility has a poor power factor below 0.8, you need a larger generator for the same kilowatt load. Power factor correction may be worth considering in those cases.
Here is a quick-reference conversion table:
| Total Load (kW) | kVA at 0.8 PF | Recommended Size with 25% Margin |
|---|---|---|
| 20 kW | 25 kVA | 31 kVA → 35 kVA |
| 50 kW | 63 kVA | 78 kVA → 80 kVA |
| 100 kW | 125 kVA | 156 kVA → 160 kVA |
| 200 kW | 250 kVA | 313 kVA → 350 kVA |
| 500 kW | 625 kVA | 781 kVA → 800 kVA |
| 1000 kW | 1,250 kVA | 1,563 kVA → 1,600 kVA |
Use our kVA to kW calculator for precise conversions at your actual power factor.
Step 3: Apply a Safety Margin
After calculating your base kVA requirement, add a safety margin. This is not a license to oversize. It is a calculated reserve for real-world conditions.
Add 20 to 25 percent to your base kVA for most industrial applications. This covers:
- Measurement inaccuracies on nameplate data
- Future load growth over three to five years
- Transient spikes from switching events
- Minor load additions between sizing and installation
For critical infrastructure like hospitals and data centers, use a 30 percent margin. These facilities cannot tolerate any capacity shortfall. For non-critical commercial loads, 20 percent is usually sufficient.
Example calculation:
- Total running load: 160 kW
- Largest starting surge: 80 kW
- Peak kW: 240 kW
- Base kVA at 0.8 PF: 240 ÷ 0.8 = 300 kVA
- With 25% margin: 300 × 1.25 = 375 kVA minimum
- Select next standard size: 400 kVA generator set
Step 4: Account for Environmental Derating
Manufacturers rate generators at sea level and 25 degrees Celsius. Most industrial sites differ from these conditions. You must derate your calculated size to match reality.
Altitude Derating
Diesel engines need oxygen for combustion. At higher altitudes, thinner air reduces both engine power and alternator cooling capacity.
Expect approximately 3.5 percent power loss per 300 meters (1,000 feet) above sea level. A generator rated at 500 kVA at sea level produces roughly 440 kVA at 1,500 meters elevation.
| Altitude | Approximate Derating |
|---|---|
| Sea level | 0% |
| 500 m (1,640 ft) | ~6% |
| 1,000 m (3,280 ft) | ~12% |
| 1,500 m (4,920 ft) | ~18% |
| 2,000 m (6,560 ft) | ~23% |
Always check the manufacturer-specific derating curve. Different engine designs respond differently to altitude.
High Temperature Effects
Ambient temperatures above 40 degrees Celsius reduce radiator efficiency and alternator output. In hot climates, derate by approximately 10 to 20 percent depending on the cooling system design.
Fuel Quality Impact
Low-cetane or contaminated diesel reduces combustion efficiency by 8 to 12 percent. In regions with inconsistent fuel quality, size with additional margin or specify engines designed for lower-grade fuel.
Step 5: Factor in Motor Starting and Voltage Dip
This step often determines the final generator size. A facility with modest running loads but large motors may need a generator two or three times larger than the running load alone would suggest.
Why Motors Decide Generator Size
When a motor starts, it behaves like a short circuit for a brief moment. The inrush current can reach two to seven times the motor’s running current. This surge demands both power (kVA) and reactive support from the generator.
The generator must maintain voltage within acceptable limits during this surge. Voltage dip limits vary by application:
- Hospitals and data centers: Maximum 10 percent voltage dip
- General industrial: Maximum 15 percent voltage dip
- Non-critical commercial: Maximum 20 percent voltage dip
Exceed these limits and your equipment malfunctions or shuts down. Downtime follows immediately.
DOL vs. Soft Starter vs. VFD
Your starting method dramatically changes the surge requirement:
| Starting Method | Starting Current Multiple | Impact on Generator Sizing |
|---|---|---|
| Direct-On-Line (DOL) | 6× to 7× running current | Largest generator required |
| Star-Delta | 3× to 4× running current | Moderate reduction |
| Soft Starter | 2.5× to 4× running current | Significant reduction |
| Variable Frequency Drive (VFD) | 1× to 1.5× running current | Minimal surge impact |
The operations manager in Peru made her decision between two options. Her mining facility had three 75 kW dewatering pumps. She required a 1,250 kVA generator because she used DOL starting method.
Her requirement decreased to 800 kVA after she installed soft starters on two pumps and used staggered start times. The soft starters generated enough savings during the first year to cover their total cost because they reduced fuel consumption and required smaller generator equipment.
Load Sequencing to Reduce Surge
If you cannot add soft starters, sequence your motor starts. Start the largest motor first while other loads are offline. Allow the generator to stabilize before starting the next motor. This spreads the surge over time instead of stacking it into a single peak.
Modern generator control systems can automate this sequencing. Our smart control systems include programmable load-shedding and staged startup logic for complex facilities.
Common Diesel Generator Sizing Mistakes to Avoid
After 25 years of manufacturing generator sets, we have seen the same errors repeat across industries and continents. Here are the five mistakes that cause the most expensive problems.
Undersizing and Ignoring Inrush
The most dangerous mistake is calculating only running load. Motors, compressors, and HVAC systems need surge capacity. Ignore it and your generator fails precisely when you need it most — during a power outage when everything tries to restart at once.
Oversizing “Just to Be Safe”
Bigger is not better. When a generator operates under 30 percent load for extended times, it starts developing wet stacking problems.
The exhaust system accumulates unburned fuel and carbon particles. The engine efficiency decreases. The expenses for maintenance work increase. The system requires a complete overhaul. The solution requires a high funding amount.
David, who worked as a facilities director in Kenya, required a 500 kVA generator to handle a 120 kW power demand. He needed “space for future development.” Wet stacking problems blocked his diesel particulate filter after 18 months. The repair expenses surpassed the value difference between his oversized equipment and the actual 200 kVA generator that he should have used.
Equipment should operate at 70 to 80 percent load during standard work activities. This range represents the optimal condition for achieving fuel efficiency while extending engine lifespan and maintaining emission controls.
Assuming 100 Percent Simultaneous Load
Not every device runs at full power at the same time. Industrial facilities use diversity factors to account for this. A typical mixed industrial load has a diversity factor of 0.8 to 0.9. This means the actual peak demand is 80 to 90 percent of the sum of all individual nameplate ratings.
Analyze your operating patterns. Use measured data from power analyzers rather than theoretical maximums. This alone can reduce your generator requirement by 10 to 20 percent without any risk.
Neglecting Site Conditions
Buyers who size at sea-level ratings for high-altitude sites or ignore ambient temperature create hidden shortfalls. The generator that works perfectly in the factory test cell may fail at your site. Always apply derating curves for your actual conditions.
Confusing Standby, Prime, and Continuous Ratings
These three ratings describe how long and how hard a generator can run. Using the wrong rating voids warranties and shortens engine life.
- Standby: Emergency power only, limited annual hours, variable load
- Prime: Primary power with variable load, unlimited hours
- Continuous: Constant load for extended periods, unlimited hours
Sizing by Application: Typical Industrial Scenarios
Different industries have different load profiles, duty cycles, and criticality requirements. The table below shows typical sizing ranges based on our field experience.
| Application | Typical Running Load | Key Sizing Factor | Common Generator Range |
|---|---|---|---|
| Manufacturing Factory | 200–1,000 kW | Motor starting surge | 350–1,500 kVA |
| Hospital / Healthcare | 150–800 kW | Life safety + fire pumps | 300–1,200 kVA |
| Data Center | 500–5,000+ kW | N+1 redundancy requirement | 1,000–8,000+ kVA |
| Mining Operation | 300–2,000 kW | Altitude derating + pumps | 600–3,000 kVA |
| Construction Site | 50–300 kW | Mobile/portable requirement | 80–500 kVA |
Manufacturing Factory
Factories combine resistive heating, motor-driven machinery, and lighting. The largest motors usually dominate sizing. A food processing plant with multiple refrigeration compressors may need 40 percent more kVA than its running load suggests.
Hospital / Healthcare
Hospitals must start life safety systems within 10 seconds per NFPA 110. Fire pumps with DOL starting create massive inrush. Size for the worst-case emergency scenario, not normal operating load.
Data Center
Data centers require N+1 or 2N redundancy. Size your generator array so that if one unit fails, the remaining units still carry the full load. Our parallel-capable generator sets support synchronized multi-unit installations up to 4,000 kVA per unit.
Mining Operation
Mines operate at altitude with heavy pump and ventilation loads. Altitude derating plus dust filtration losses can reduce effective capacity by 25 to 30 percent. Size accordingly.
Construction Site
Construction loads are temporary and mobile. Trailer-mounted or containerized generators offer flexibility. Size for the peak tool load plus welding equipment surge.
When to Consult a Manufacturer for Custom Sizing
Simple facilities with stable, well-documented loads can use the process above. Complex facilities benefit from professional load analysis. Contact a manufacturer when:
- Your facility has 10 or more motors above 30 kW
- You operate at altitude above 1,000 meters or temperature above 40 degrees Celsius
- Your application is life safety critical (hospitals, emergency services)
- You need parallel operation or future expansion capability
- Your load includes non-linear equipment like VFDs or UPS systems that create harmonic distortion
Prepare these documents before your consultation:
- Complete load inventory with nameplate data
- Single-line electrical diagram
- Site elevation and maximum ambient temperature
- Operating schedule showing which loads run simultaneously
- Future expansion plans for the next three to five years
- Starting method for each motor (DOL, soft starter, VFD)
At Shandong Huali Electromechanical Co., Ltd., our 80-engineer team uses transient simulation software and data from our 20 MW national-standard testing center to verify every sizing recommendation. We do not guess. We calculate, test, and validate before you commit.
Our diesel generator sets range from 10 kVA to 4,000 kVA with Cummins, Perkins, Weichai, and Yuchai engines. All units complete complete pre-delivery testing at 100 percent according to ISO 9001 and CE-certified procedures.
Conclusion
Choosing the right diesel generator size is engineering, not estimation. Start with a complete load inventory.
Calculate running watts plus the largest starting surge. Convert to kVA using your actual power factor. Apply a 20 to 25 percent safety margin. Then derate for altitude, temperature, and fuel quality.
Avoid the two extremes. Undersizing causes failures during your most critical moments. Oversizing wastes capital and destroys engines through wet stacking. The right size operates at 70 to 80 percent load, delivers reliable power, and leaves room for controlled growth.
For complex industrial facilities, professional load analysis pays for itself many times over. A correctly sized generator runs longer, costs less to maintain, and protects your operations when the grid fails.
Contact Shandong Huali today for expert generator sizing and a customized power solution built for your exact requirements.
