A step-by-step sizing guide from someone who has seen hundreds of undersized generators fail — and just as many oversized ones waste fuel like there is no tomorrow.

Let me tell you a story that still bothers me. A few years ago, a client in Nairobi ordered a 500 kW Yuchai generator for a shopping mall that only needed about 120 kW. Their logic? “Better to have too much than too little.”

Result: the unit ran at 25% load for most of its life. Wet stacking destroyed the exhaust valves within 2,800 hours. Fuel bills were nearly double what they should have been. And when they finally called us at Tesla Power for help, the overhaul cost was north of $18,000.

Getting the size right is not optional. It is the single most important decision you will make about your generator. This guide will walk you through exactly how to figure it out — no engineering degree required.

Why Getting the Size Wrong Is So Expensive

Before we get into the math, let me explain why diesel generator sizing matters so much:

Undersized Generator

• Circuit breakers trip repeatedly
• Motor starting failures
• Engine overheating and premature failure
• Voltage instability damages sensitive equipment
• Constant overload alarms

Oversized Generator

• Wet stacking and carbon buildup
• 30-40% higher fuel cost per kWh
• Poor combustion efficiency
• Higher purchase price wasted on unused capacity
• Increased cylinder wear from low-load operation

The sweet spot? Your generator should run at 60-80% of its rated capacity during normal operation. That is where Yuchai diesel engines deliver their best fuel economy, cleanest combustion, and longest component life.

Understanding the Numbers: kW vs. kVA

Before you can size anything, you need to understand the two fundamental power ratings:

• kW (kilowatts): Real, usable power. This is what your equipment actually consumes. A 10 kW air conditioner needs 10 kW to run.
• kVA (kilovolt-amperes): Apparent power. This is what the generator produces. The relationship between kW and kVA is determined by the power factor (PF).

kW = kVA x Power Factor
kVA = kW / Power Factor

For most diesel generators: Power Factor = 0.8
Example: A 125 kVA generator delivers 100 kW of usable power (125 x 0.8)

Most Yuchai diesel generator sets from Tesla Power are rated at a 0.8 power factor, which is the industry standard for commercial and industrial applications.

Step-by-Step: How to Calculate Your Generator Size

Here is the process I walk every client through. Grab a pen, a spreadsheet, or use the free tool I will mention below.

Step 1: List Every Electrical Load

Walk through your facility and write down every single piece of equipment that needs to run on generator power. Do not forget:

• Lighting (all types — LED, fluorescent, HID)
• HVAC systems (compressors, air handlers, fans)
• Motors (pumps, conveyors, elevators, compressors)
• Electronics (computers, servers, POS systems)
• Kitchen equipment (if applicable)
• Security systems (cameras, access control, alarms)
• Fire safety systems (fire pumps, smoke extraction fans)
• Water heaters and boilers

Step 2: Determine Running vs. Starting Power

This is where most people make their first mistake. Electric motors draw 3-7 times their running current during startup. A 15 kW water pump might need 60 kW just to get spinning — but only for 2-5 seconds.

For each motor, note both its running power (in kW) and its starting power (running kW multiplied by the lock-rotor code factor, typically 3-5x for standard motors).

Practical shortcut: Not all motors start at the same time. If you can stagger motor starts (which a good control system does automatically), you only need to account for the single largest motor starting while all others are running. This can significantly reduce your required generator size.

Step 3: Apply Diversity Factor

In most facilities, not everything runs simultaneously. The lights might be on, but the air conditioning compressor cycles on and off. The kitchen equipment runs in shifts. This is called load diversity.

Total Connected Load = Sum of ALL equipment running watts
Diversified Load = Total Connected Load x Diversity Factor

Typical Diversity Factors:
– Office buildings: 0.6 – 0.7
– Retail: 0.7 – 0.8
– Industrial: 0.8 – 0.9
– Hospitals/Data centers: 0.9 – 1.0

Step 4: Add Safety Margin and Derating

Take your calculated load and add a 20-25% margin for:

• Future equipment additions
• Altitude derating (3% per 300m above sea level)
• Temperature derating (1-2% per 10 degrees Celsius above 25 degrees)
• Power factor differences between calculated and actual loads

Step 5: Convert to Generator Rating

Required Generator kVA = (Diversified Load + Safety Margin) / 0.8

Example:
– Diversified load: 85 kW
– Safety margin (20%): 85 x 1.2 = 102 kW
– Required kVA: 102 / 0.8 = 127.5 kVA
– Select: 150 kVA generator (round up to next standard size)

Quick Reference: Yuchai Generator Sizes by Application

Not everyone wants to do the math. Here is a quick reference guide based on the most common applications I encounter:

Application
Typical Load
Recommended Yuchai Size
Engine Model

Small office / shop (backup)
10-30 kW
30-50 kVA
YC4D

Restaurant / cafe
30-60 kW
50-80 kVA
YC4D / YC6B

Medium office building
60-120 kW
100-150 kVA
YC6B

Shopping mall (medium)
100-250 kW
150-312 kVA
YC6MK

Small factory
200-400 kW
250-500 kVA
YC6MK / YC6T

Large factory / mining
400-800 kW
500-1000 kVA
YC6T / YC16VC

Telecom tower
5-20 kW
15-30 kVA
YC4D

Construction site
50-150 kW
60-200 kVA
YC4D / YC6B

Important: These are starting points, not final answers. Your actual needs depend on your specific equipment mix, climate, and operating requirements. Tesla Power offers free site assessments and detailed load calculations — because getting this right is too important to guess at.

Tools That Make Sizing Easier

You do not have to do all this by hand. Here are the tools I recommend:

• Tesla Power Load Calculator: Contact us for access to our proprietary load calculation tool. It handles motor starting calculations, diversity factors, and environmental derating automatically. Free for prospective clients.
• Caterpillar Sizing Tool (online): While designed for Cat generators, the calculations are universal. Useful for double-checking your own numbers.
• Automatic Transfer Switch (ATS) with load management: Modern ATS units can prioritize and sequence loads automatically. This means you can use a smaller generator by controlling which loads start when. Tesla Power supplies ATS systems with built-in load management capabilities.
• Power analyzers and loggers: For existing facilities, install a power logger on your main panel for 1-2 weeks. This gives you actual demand data instead of estimates. Fluke and Dranetz make excellent units.

4 Sizing Mistakes I See All the Time

After sizing generators for over a decade, I can tell you that the same mistakes come up repeatedly. Learn from other people’s expensive lessons:

Mistake 1: Adding up nameplate ratings without diversity factor.
A factory with 300 kW of nameplate equipment rarely draws more than 180-200 kW at any given moment. Using the full 300 kW means buying a generator that is 50% larger than necessary. Cost difference? Tens of thousands of dollars.

Mistake 2: Ignoring motor starting requirements.
The running load of your facility might be 80 kW, but if you have a 30 kW compressor that starts across-the-line, your generator needs to handle a momentary surge of 90-150 kW. If the generator cannot supply that surge, the motor will not start — or the voltage dip will crash every computer in the building.

Mistake 3: Forgetting about altitude and temperature.
A generator rated at 200 kW at sea level and 25 degrees Celsius will only deliver about 172 kW at 1,500 meters altitude and 40 degrees Celsius. That 14% derate catches people off guard, especially in highland areas of East Africa, the Andes, and parts of Central Asia.

Mistake 4: Not planning for growth.
You build your generator room around a 100 kW unit, and two years later you add a new production line that needs 40 kW more. Now your generator is overloaded, and there is no physical space for a larger unit. Always build in 20-25% headroom and make sure your installation can accommodate an upgrade.

Tesla Power Yuchai Generator Product Range

Here is an overview of the Yuchai diesel generator selection available from Tesla Power:

• 30-60 kVA (24-48 kW): YC4D engine, ideal for small commercial, retail backup, and residential compounds. Compact, fuel-efficient, and easy to maintain.
• 80-150 kVA (64-120 kW): YC6B engine, the workhorse for medium offices, restaurants, and small industrial applications. Excellent reliability track record.
• 200-312 kVA (160-250 kW): YC6MK engine, designed for shopping malls, mid-size factories, and hotels. Features advanced fuel injection and electronic governor control.
• 400-625 kVA (320-500 kW): YC6T engine, built for heavy industrial use, mining, and large manufacturing. Reinforced block construction for extended continuous operation.
• 800-1250 kVA (640-1000 kW): YC16VC engine, the flagship series for major industrial installations, large-scale mining, and critical infrastructure. V16 configuration delivers smooth, high-power output.

Every unit includes: Stamford or Leroy-Somer alternator, Deep Sea controller, heavy-duty radiator, base fuel tank, circuit breaker, battery and charger, and full documentation package.

Available configurations: Open type, soundproof canopy (75-85 dBA), containerized (for harsh environments), and mobile trailer-mounted.

Service and support: Tesla Power provides complete turnkey solutions including site survey, electrical design, installation, commissioning, operator training, and ongoing maintenance contracts. We maintain spare parts inventory for all Yuchai models and offer 24/7 emergency response in our service regions.

Prime Power vs. Standby: Does It Change Sizing?

Yes — and this is another common source of confusion. A generator’s rating depends on its intended use:

• Standby Power (ESP): Maximum power available during utility outage. No overload capability. Typical usage: less than 200 hours per year, with maximum 500 hours per year.
• Prime Power (PRP): Unlimited running hours at variable load. Can handle 10% overload for 1 hour every 12 hours. Use for continuous or near-continuous operation.
• Continuous Power (COP): Constant load, unlimited hours. No overload capability. Used for base-load applications where the generator runs at a fixed load 24/7.

Rule of thumb: A 100 kW standby-rated generator typically has a prime power rating of about 90 kW. If your application requires continuous operation, always size based on the prime power rating — never the standby rating. Tesla Power clearly labels both ratings on every unit we supply.

The Bottom Line

Sizing a Yuchai diesel generator is not rocket science, but it does require careful attention to detail. List your loads. Account for motor starting. Apply diversity factors. Add safety margin. Derate for altitude and temperature. Round up to the next standard size.

Get it right, and your generator runs efficiently for 15-25 years. Get it wrong, and you will pay for that mistake every single month in fuel, maintenance, and premature replacement costs.

When in doubt, ask for help. A free consultation with Tesla Power could save you more money than you might imagine.

Frequently Asked Questions

What size Yuchai generator do I need for a 3-bedroom house?

For a typical 3-bedroom home running lights, fans, a refrigerator, TV, and a small AC unit, a 15-20 kVA (12-16 kW) Yuchai generator is usually sufficient. If you want to run multiple AC units, a water heater, and an electric stove simultaneously, you may need 30-40 kVA. The best approach is to list all appliances and their wattages, then add 20% margin.

Can I run a 200 kW load on a 200 kVA generator?

No. A 200 kVA generator at 0.8 power factor delivers 160 kW of usable power. Running a 200 kW load would overload the generator by 25%, which would quickly trigger protective shutdowns and could cause engine damage. You would need at least a 250 kVA generator for a 200 kW load.

How does altitude affect Yuchai generator sizing?

As altitude increases, air density decreases, which means the engine cannot burn as much fuel and produces less power. The standard derate is approximately 3% for every 300 meters (1,000 feet) above sea level. A 200 kW generator at sea level would only produce about 172 kW at 1,500 meters altitude. Tesla Power factors this into every sizing recommendation.

Should I size my generator for peak load or average load?

You need to size for your peak load — the maximum power demand at any given moment, including the largest motor starting while other equipment runs. However, with a smart ATS system that sequences motor starts, you can reduce this peak significantly. The generator should then operate at 60-80% load during normal conditions for optimal efficiency and longevity.

What happens if my generator is slightly undersized?

A generator that is 5-10% undersized may run but will experience frequent overload alarms, voltage dips, and potential overheating. During motor starting events, the voltage may drop low enough to trip sensitive electronics. Chronic overloading accelerates engine wear and dramatically reduces diesel generator service life. If your calculations show you are right on the border between two sizes, always go up.

About the Author: This guide is based on the author’s experience sizing, supplying, and commissioning diesel generators for commercial, industrial, and residential applications across Africa and Asia. The examples and calculations reflect real project scenarios and have been verified against actual performance data. For a free sizing consultation, contact Tesla Power with your facility details and load information.