Choosing a diesel generator is one of those decisions that seems straightforward until you actually start researching. Suddenly you’re drowning in specifications\u2014kW, kVA, prime vs standby ratings, voltage regulation, THD, power factor. And that’s before considering Yuchai’s dozens of engine models, alternator options, and enclosure types.\n\nLast month, I consulted on a project where the facility manager had specified a 600 kW generator for his manufacturing plant. After analyzing his actual loads, we discovered he needed 380 kW\u2014saving him $62,000 in equipment costs plus thousands annually in unnecessary fuel consumption.\n\nHow do you select the right Yuchai diesel generator without overpaying for capacity you don’t need or under-sizing and risking failures? Let’s break down the selection process step by step.\n\n## Step 1: Understand Your Power Requirements\n\nThis is where most selection errors occur. You can’t choose the right generator until you know exactly what you need to power.\n\n### Identify Your Critical vs. Non-Critical Loads\n\nNot everything needs backup power. Start by categorizing:\n\nCritical loads (must run): \n- Life safety systems (fire alarms, emergency lighting, exit signs) \n- Essential HVAC (server room cooling, critical area ventilation) \n- Computers, servers, network equipment \n- Medical equipment \n- Refrigeration for perishable goods or materials \n- Security systems\n\nNice-to-have loads (desirable but not essential): \n- General office lighting \n- Non-critical HVAC zones \n- Break room appliances \n- Landscape lighting\n\nNon-essential loads (can be shed): \n- Water heaters \n- Electric vehicle chargers \n- Decorative lighting \n- High-load equipment that can be temporarily shut down\n\nYou might decide to power only critical loads (smaller, less expensive generator) or critical plus nice-to-have loads (larger generator but more operational continuity).\n\n### Calculate Your Connected Load\n\nList every electrical device/circuit you’ll power. For each item, note:\n\nRunning watts (kW): The continuous power consumption during operation.\n\nStarting watts (surge): Many devices draw 2-10x their running power during startup, especially motors.\n\nDuty cycle: Will this run continuously or intermittently?\n\nCommon equipment power requirements:\n\n- Computer workstation: 0.3-0.5 kW \n- LED lighting (per fixture): 0.02-0.1 kW \n- Server rack: 2-8 kW \n- Small office HVAC (5-ton): 6-8 kW running, 18-30 kW starting \n- Walk-in cooler/freezer: 3-10 kW running, 10-30 kW starting \n- Elevator: 10-40 kW running, 30-100 kW starting\n\nTotal connected load = Sum of all running watts\n\nHowever, this isn’t your generator size yet!\n\n### Apply Demand and Diversity Factors\n\nNot all loads run simultaneously. Applying realistic diversity factors prevents massive over-sizing:\n\nLighting: 80-100% (most lights on when power fails) \nReceptacles (outlets): 40-60% (not all equipment plugged in and running) \nHVAC: 80-90% (depends on season and zoning) \nMotors: Calculate based on which actually start simultaneously\n\nFor a typical office building:\n\n- Connected load: 500 kW \n- After diversity factors: 325-375 kW actual demand\n\nDon’t guess at diversity factors\u2014examine your utility bills to see actual peak demand. This is the most reliable indicator of real-world power needs.\n\n> [TODO: Continue sections]”}]

Account for Motor Starting Currents

This is the calculation that trips up most buyers. Motors, compressors, and other inductive loads require massive surge current during startup—often 6-10x their running power.

Example: A 10 HP (7.5 kW) HVAC compressor motor:

• Running load: 7.5 kW
• Starting surge: 45-60 kW for 2-5 seconds

If you simply add up running loads without accounting for starting surges, your generator will be catastrophically undersized. The voltage will collapse when motors start, potentially damaging equipment or preventing startup altogether.

Solution: Use “stepped starting” in your calculations. Determine which motors must start simultaneously vs. sequentially:

Scenario 1 (simultaneous start):

• Base loads: 150 kW
• HVAC 1 starting: 45 kW surge
• HVAC 2 starting: 45 kW surge
• Elevator starting: 80 kW surgePeak requirement: 320 kW

Scenario 2 (sequenced start with 30-second delays):

• Base loads: 150 kW
• HVAC 1 starting: 45 kW surge (then drops to 7.5 kW running)
• After 30 sec, HVAC 2 starting: 45 kW surge
• After another 30 sec, elevator starting: 80 kWPeak requirement: 230 kW

Manufacturers like Tesla Power can help program load sequencing into your controller to prevent simultaneous motor starts.

Step 2: Choose Between Prime and Standby Power Ratings

This confuses many buyers, yet it’s critical for proper generator selection and longevity.

Standby Power Rating (ESP)

Definition: Maximum power available for emergency use only, typically limited to 200 hours per year with variable load.

When to use: True emergency generator applications where utility power is reliable and generator runs only during rare outages (hospitals, data centers with grid power, most commercial facilities).

Duty cycle: Intermittent operation, primarily for testing and actual emergencies.

Overload capacity: Typically can handle 10% overload for 1 hour in every 12 hours of operation.

Prime Power Rating (PRP)

Definition: Maximum power available for unlimited hours per year at variable load, with typical loading at 70-80% of rated capacity.

When to use: Locations without reliable grid power, construction sites, continuous industrial operations, prime power applications where the generator is the primary power source.

Duty cycle: Continuous or near-continuous operation.

Overload capacity: Limited overload capability—designed to run at rating for extended periods.

Continuous Power Rating (COP)

Definition: Power available 24/7/365 at constant 100% load (rare for most applications).

When to use: Base-load power plants, continuous industrial processes with non-varying loads.

Reality check: Very few applications actually need continuous rating. Prime rating is appropriate for most “continuous” applications since real-world loads vary.

The Ratings Gap

Here’s what’s critical: The same generator has different ratings depending on duty.

Example: A typical 500 kW Yuchai diesel generator:

• Standby (ESP): 500 kW
• Prime (PRP): 450 kW (10% lower)
• Continuous (COP): 400 kW (20% lower)

Common mistake: A salesperson quotes you a “500 kW generator” for a prime power application. You assume you have 500 kW available continuously. In reality, that unit is only rated for 450 kW prime power. Running it continuously at 500 kW will cause premature engine wear and failures.

Always verify which rating applies to your application and confirm quotes specify the correct rating.

Step 3: Determine Required Runtime and Fuel Tank Sizing

How long does your generator need to run between refueling?

Calculate Fuel Consumption

Yuchai diesel engines typically consume 0.22-0.28 liters per kWh (0.06-0.074 gallons per kWh) depending on engine size and load factor.

Example calculation: 250 kW Yuchai diesel generator running at 75% load (188 kW average)

• Consumption rate: 0.25 L/kWh
• Hourly fuel use: 188 kW × 0.25 = 47 liters/hour (12.4 gallons/hour)

Size Your Fuel Tank

Minimum runtime targets by application:

• Residential/small commercial: 8-12 hours (to cover overnight outages)
• Medium commercial: 24-48 hours (until utility restoration or refueling possible)
• Critical facilities (hospitals, data centers): 48-72 hours minimum, often 96+ hours
• Remote/prime power: 7+ days between refueling

Our 250 kW example needing 48-hour runtime:

• 47 L/hour × 48 hours = 2,256 liters (596 gallons)
• Add 20% safety margin: 2,707 liters (715 gallons)

Tank sizing reality: Fuel tanks are expensive. A 750-gallon diesel tank costs $4,000-$8,000 installed. Don’t over-specify runtime you don’t need, but don’t cut corners on critical applications.

Alternative: Install a larger base tank with capacity for refueling during extended outages rather than sizing the tank for worst-case scenarios.

Step 4: Select Voltage, Frequency, and Phase Configuration

Your electrical system dictates these specifications.

Voltage Selection

North America typical:

• Residential: 120/240V single-phase
• Small commercial: 120/208V or 120/240V three-phase
• Large commercial/industrial: 277/480V three-phase

International typical:

• Residential: 230V single-phase
• Commercial: 400V three-phase
• Some regions: 380V, 415V, 440V three-phase

Critical: Voltage must match your facility. Trying to adapt a 480V generator to a 208V facility requires expensive transformers and introduces efficiency losses.

Frequency

North America, parts of South America, Japan: 60 HzEurope, most of Asia, Africa, Oceania: 50 Hz

Yuchai engines can be configured for either frequency, but this must be specified at ordering. The engine RPM changes (1500 RPM for 50 Hz, 1800 RPM for 60 Hz), so this isn’t field-adjustable.

Phase Configuration

Single-phase: One hot wire, one neutral. Simple loads (lighting, small motors, residential). Limited to smaller generators (<30 kW typically).

Three-phase: Three hot wires, one neutral. More efficient for motors, allows larger capacities, standard for commercial/industrial. Nearly all commercial generators above 50 kW are three-phase.

Your facility’s electrical panel determines this. If you have three-phase distribution, you need a three-phase generator.

Step 5: Evaluate Alternator Quality and Specifications

The alternator determines power quality. All Yuchai diesel generators use Yuchai engines, but alternator choices vary dramatically.

Key Alternator Specifications

Voltage regulation: How tightly voltage stays within acceptable range under varying loads.

• Budget alternators: ±8-15% regulation
• Mid-tier: ±3-5% regulation
• Premium: ±1-2% regulation

For sensitive electronics, better regulation matters. Data centers and medical facilities need premium alternators.

THD (Total Harmonic Distortion): Measure of power quality with non-linear loads (computers, LED lighting, VFD motors).

• Budget: 10-15% THD
• Mid-tier: 5-8% THD
• Premium: <5% THD

High THD damages electronics and causes overheating. Again, critical facilities need low THD.

Reputable alternator brands:

• Stamford (UK, premium quality)
• Marathon (US, excellent mid-tier to premium)
• Leroy-Somer (France, premium)
• Mecc Alte (Italy, quality mid-tier)
• Engga (China, acceptable mid-tier)

Manufacturers like Tesla Power offer alternator choices. Don’t just accept the cheapest option—specify quality appropriate to your application.

Brushless vs. Brush-Type Alternators

Brushless (recommended): No physical contacts to wear out, automatic voltage regulation, lower maintenance, longer life. Standard on quality diesel generators.

Brush-type (avoid except portable units): Physical carbon brushes wear and require replacement every 2,000-5,000 hours. Cheaper upfront, expensive over time. If someone’s selling you a brush-type alternator for a permanent installation, they’re cutting corners.

Step 6: Choose Enclosure Type Based on Environment and Noise

Your installation environment dictates enclosure requirements.

Open Frame

Description: Generator mounted on steel base frame with no enclosure.

Advantages: Least expensive, easy access for maintenance, excellent cooling airflow.

Disadvantages: No weather protection, loudest option (90-105 dB), no security.

Best for: Indoor installations in mechanical rooms with adequate ventilation, where noise isn’t a concern.

Weather-Resistant Enclosure

Description: Steel or aluminum enclosure protecting from rain, snow, UV, but minimal acoustic treatment.

Noise level: 75-85 dB @ 7 meters

Advantages: Outdoor installation capability, reasonable cost (15-25% premium over open frame).

Disadvantages: Still quite loud for residential or noise-sensitive areas.

Best for: Industrial outdoor installations, remote locations, areas without strict noise ordinances.

Sound-Attenuated Enclosure

Description: Engineered enclosure with acoustic foam, baffles, and sound-dampening construction.

Noise level: 65-75 dB @ 7 meters

Advantages: Quiet enough for most commercial settings, weather-resistant.

Cost: +35-50% vs. open frame

Best for: Urban commercial installations, locations near residences, facilities with noise restrictions.

Super-Silent Enclosure

Description: Advanced acoustic engineering with multiple sound-dampening layers.

Noise level: 55-65 dB @ 7 meters (comparable to normal conversation)

Advantages: Extremely quiet, suitable for hospitals, residential areas, hotels.

Cost: +55-80% vs. open frame

Best for: Premium applications where noise is critical, dense urban environments, luxury facilities.

Reality check: Acoustic treatment significantly increases cost. A 200 kW generator might be $42,000 open-frame or $68,000 super-silent. Only pay for the noise reduction you actually need.

Step 7: Consider Controller Features and Automation

The controller is your generator’s brain, managing start/stop, load transfer, protection, and monitoring.

Essential Controller Functions

Auto-start capability: Detects utility power failure and automatically starts generator within 10-30 seconds.

Load transfer coordination: Works with your automatic transfer switch (ATS) to safely switch loads from utility to generator and back.

Engine protection: Monitors and shuts down for low oil pressure, high coolant temperature, overspeed, and other fault conditions.

Basic instrumentation: Voltage, frequency, current, oil pressure, coolant temperature, running hours.

All reputable diesel generator systems include these basic functions.

Advanced Controller Features

Remote monitoring: Cloud connectivity allowing you to monitor generator status from anywhere via phone/computer. Essential for critical facilities or remote installations.

Multiple gen-set synchronization: Paralleling multiple generators to handle large or varying loads. Complex applications require sophisticated controllers.

Load management: Priority load shedding to match available generator capacity. Useful when generator is undersized for total connected load—it automatically drops non-critical loads.

Utility grid synchronization: For selling power back to grid or seamless transfer. Requires utility-grade synchronization features.

Data logging and reporting: Records performance data, fault history, maintenance schedules.

Controller Brands

Three brands dominate quality installations:

Deep Sea Electronics (DSE): UK-based, excellent reliability, strong in standby/emergency applications, comprehensive model range.

DEIF: Danish, sophisticated paralleling and synchronization, preferred for complex multi-generator systems.

ComAp: Czech, versatile, excellent for rental/mobile applications, good balance of features and cost.

Manufacturers like Tesla Power typically offer all three brands. Your application determines which is most appropriate.

Step 8: Factor in Site Conditions and Deration

Generator ratings assume standard conditions. Real-world installations often require derating.

Altitude Deration

Diesel engines lose power at high altitude due to lower air density.

Deration factors:

• Sea level to 1,000m (3,300 ft): No deration
• 1,000-2,000m (3,300-6,600 ft): -3% per 100m above 1,000m
• Above 2,000m: Additional engineering required

Example: 500 kW generator at 1,500m altitude

• Altitude difference from baseline: 500m
• Deration: 500m ÷ 100m × 3% = 15%
• Effective capacity: 500 kW × 0.85 = 425 kW

If you need 500 kW at altitude, you must purchase a larger generator.

Ambient Temperature Deration

Standard ratings assume 25-40°C (77-104°F) ambient temperature.

Hot climates: Above 40°C, derate approximately 4% per 5°C.

Cold climates: Below -10°C, cold-start capabilities become critical. You may need block heaters, battery warmers, and cold-weather fuel.

Duty Cycle and Load Factor

Running a generator continuously at 100% load causes premature wear. Optimal loading is 70-85% of rated capacity.

If your actual load is 300 kW continuous, don’t buy a 300 kW (prime) generator. Buy 350-375 kW so you operate at optimal loading.

Making Your Final Selection

You’ve gathered your requirements:

✓ Calculated load with proper diversity and motor starting allowances✓ Determined prime vs. standby rating needed✓ Sized fuel capacity for required runtime✓ Verified voltage, frequency, and phase match your facility✓ Selected alternator quality for your power quality needs✓ Chosen enclosure for your environment and noise requirements✓ Specified controller features for your operational needs✓ Applied derating for altitude, temperature, and duty cycle

Now you can request quotes with confidence. You’re specifying a Yuchai diesel generator correctly matched to your actual needs—not over-paying for excess capacity or under-buying and risking failures.

Work with technically competent manufacturers like Tesla Power who can validate your calculations and recommend optimal configurations. The selection process takes time upfront but ensures decades of reliable backup power.

Frequently Asked Questions

What size Yuchai diesel generator do I need for my facility?

Calculate your total critical load, apply realistic diversity factors (typically 60-80%), and add surge capacity for your largest motor starting current. For standby applications, this gives you the required standby generator capacity. For prime applications, upsize by 15-20% so you operate at optimal 70-85% loading. As a rough guide: small offices need 20-75 kW, medium commercial facilities need 100-300 kW, and large industrial facilities need 400+ kW. Always consult with manufacturers for proper sizing—errors cost thousands.

Should I choose prime or standby power rating?

Choose standby rating if you have reliable utility power and the generator runs only during rare outages (typically <200 hours/year). Choose prime rating if the generator is your primary power source or runs frequently (>500 hours/year). The same physical generator has different ratings—a 500 kW standby unit is typically only 450 kW prime. Using a standby-rated generator for prime duty causes rapid wear and premature failure. When in doubt, specify prime rating—it provides more conservative capacity.

How do I account for motor starting in generator sizing?

Motors require 6-10x their running power during startup. Calculate your base load, then add the starting surge of your largest motor (or combined surge if multiple motors start simultaneously). A genset controller can sequence motor starts with 30-60 second delays, dramatically reducing peak capacity requirements. For example, three 10 kW motors starting simultaneously need 180 kW surge capacity; sequenced starting needs only 60 kW surge. Quality manufacturers like Tesla Power can program load sequencing into your controller.

What’s the difference between kW and kVA ratings?

kW (kilowatts) measures real power that does actual work. kVA (kilovolt-amperes) measures apparent power including reactive components. For resistive loads (heaters, incandescent lights), kW = kVA. For inductive loads (motors, transformers), kVA is higher due to power factor. Diesel generators are typically rated in kW at 0.8 power factor, meaning a 500 kW generator provides 625 kVA. Always confirm whether quotes specify kW or kVA—and at what power factor—to ensure accurate comparisons.

How much does generator size affect fuel consumption?

Larger generators consume more fuel, but the relationship isn’t linear. A 200 kW Yuchai diesel generator at 50% load (100 kW output) consumes roughly 25-28 liters/hour. A 100 kW generator at 100% load (same 100 kW output) consumes 22-25 liters/hour—about 10-15% less because it operates at optimal efficiency. This is why proper sizing matters: oversized generators waste fuel, undersized generators risk overload. Target 70-85% loading during typical operation for best fuel economy and engine longevity.