Is it Better to Oversize or Undersize a Diesel Generator for Specific Applications? [Expert Guide]

Diesel generators remain the backbone of reliable backup and primary power supply systems in industries, commercial complexes, hospitals, construction sites, and remote locations. 

Choosing the right size of diesel generator is one of the most important decisions an organization or technician can make. 

Unfortunately, many decision-makers often fall into the trap of oversizing or undersizing their diesel generators, thinking it’s a minor compromise. 

In truth, this single decision can significantly impact fuel costs, generator lifespan, operational efficiency, and even safety.

In this in-depth guide, we’ll help you understand the effects of incorrect sizing and whether it’s better to oversize or undersize a diesel generator for your specific application.

Understanding Diesel Generator Sizing

Generator sizing is the process of determining the correct power output a generator must deliver to meet the electrical load requirements of a facility. 

Proper sizing ensures the generator can handle starting loads, continuous running loads, and any future growth.

Key factors in diesel generator sizing include:

• Total electrical load: Sum of all appliances and machines that will be powered.

• Type of load: Resistive (heaters, lights) or inductive (motors, compressors).

• Startup surge requirements: Motors and compressors require more power to start.

• Duty cycle: Will the generator operate as a standby, prime, or continuous power source?

• Location conditions: Altitude, temperature, and humidity can impact performance.

For example, a hospital’s generator must handle life-saving equipment, HVAC systems, and emergency lighting simultaneously, all with varying load types. 

Sizing for such an application requires detailed load assessment and allowances for future expansion.

What Happens If You Undersize a Diesel Generator?

Undersizing occurs when the generator’s capacity is lower than the required electrical load. 

It’s tempting to save on initial costs by buying a smaller unit, but this can lead to disastrous consequences.

Common Signs of an Undersized Generator:

• Frequent tripping or shutdowns

• Overheating

• Excessive engine wear

• Delayed or failed start of heavy equipment

Risks of an Undersized Generator:

1. Overload Shutdowns: An undersized diesel generator frequently operates beyond its rated capacity. 

This overload can trigger safety shutdowns or worse—permanent damage to the alternator and engine.

2. Overheating: Running at or above capacity continuously pushes the generator’s internal components to their limits. 

The result is overheating, which accelerates engine wear, shortens oil life, and increases the risk of breakdowns.

3. Poor Equipment Performance: When a generator can’t deliver enough starting kVA for inductive loads like motors or compressors, the equipment may fail to start or run sluggishly, reducing operational productivity.

4. Reduced Generator Lifespan: An overworked generator breaks down faster. 

It consumes more fuel and oil, requires more frequent servicing, and is likely to suffer component failure earlier than expected.

5. High Maintenance Costs: Due to continuous stress on components, maintenance becomes more frequent and expensive, wiping out any initial savings from purchasing a smaller unit.

Case Study: Data Center Failure In 2021, a small commercial data center in Lagos, Nigeria, experienced critical server outages during a power failure. 

Their 100 kVA generator was undersized for the actual 130 kVA load, leading to system crashes, overheating, and client data loss. 

The eventual solution involved replacing the generator and investing in proper load analysis.

What Happens If You Oversize a Diesel Generator?

Oversizing occurs when the generator capacity significantly exceeds the required electrical load. 

While this may seem like a safer bet, it comes with its own drawbacks.

Dangers of Oversizing:

1. Light Loading and Wet Stacking: When a diesel generator operates at less than 30% of its rated capacity, it doesn’t burn fuel efficiently. 

Unburned fuel residues accumulate in the exhaust system, causing a condition known as wet stacking. 

This results in carbon build-up, fouled injectors, and potential engine damage.

2. Reduced Fuel Efficiency: Larger generators consume more fuel, even when powering minimal loads. 

This leads to higher fuel bills without any performance advantage.

3. Increased Capital and Installation Costs: Oversized units are more expensive to purchase, install, and transport. 

These costs add up quickly for small to medium-scale operations.

4. Higher Emissions and Environmental Impact: Generators running at low load levels release more unburnt hydrocarbons and particulate matter, reducing environmental compliance.

5. Maintenance Complexity: Larger machines may require more complex maintenance, often requiring specialized parts and expertise.

Case Study: Oversized Generator in a Shopping Mall A newly developed mall in Accra, Ghana, installed a 1000 kVA diesel generator, expecting future expansions. 

For the first two years, the actual load never exceeded 300 kVA. Wet stacking, carbon build-up, and low fuel efficiency plagued the generator, costing thousands of dollars in repairs and fuel waste.

How to Correctly Size a Diesel Generator for Specific Applications

Correct generator sizing balances efficiency, reliability, and future scalability. Follow this practical guide:

Step-by-Step Sizing Process:

1. Identify All Electrical Loads: List all equipment and their power ratings (in kW or kVA). Use nameplates or consult the equipment manufacturer.

2. Separate Loads into Critical and Non-Critical: Essential systems like lighting, servers, or life-support equipment must always run during outages.

3. Calculate Total Running Load: Sum all power demands during normal operation. This gives your base load.

4. Account for Starting Loads: Inductive loads require up to 6 times their running power during startup. Add this surge capacity to your calculation.

5. Add a Buffer for Future Growth: Add 15–25% extra capacity to allow for future equipment additions or business expansion.

Application-Based Sizing Examples:

Hospitals: Require continuous power for life-saving machines. Include redundant systems. Aim for 100% backup coverage with minimum 80% load utilization.

Construction Sites: Power varies daily. Size for peak load during equipment operation and allow for future tools.

Data Centers: Require precision. Power loads are constant and critical. Generators must integrate with UPS systems.

Residential Backup: Typically smaller loads. Prioritize essential appliances and HVAC systems.

Industrial Plants: Variable load with high inductive startup. Often require multiple synchronized generators.

Which is Worse?

While both scenarios have serious drawbacks, undersizing often results in immediate operational failures and costly downtime. 

Oversizing, while inefficient, is less likely to result in critical shutdowns but leads to long-term maintenance and fuel costs.

Ideal Operating Range

Experts recommend sizing a generator so that it typically runs at 70% to 80% load for optimal performance, fuel economy, and lifespan.

Best Practices for Diesel Generator Sizing

1. Work With Qualified Professionals: Generator suppliers and electrical consultants offer load analysis services. Always engage experts for critical infrastructure.

2. Use Load Banks: If your generator is oversized, use load banks to simulate additional load and prevent wet stacking.

3. Monitor and Log Load Profiles: Modern systems let you track load trends over time. Use this data to optimize generator performance.

4. Plan for Modular Expansion: Rather than one large generator, consider using two or more smaller units in parallel. This allows flexibility and better load matching.

5. Don’t Rely on Guesswork: Avoid estimating load without proper tools. Use generator sizing calculators or professional tools.

FAQs

Can I parallel two smaller generators instead of one large one? Yes. Paralleling generators provides flexibility, scalability, and better efficiency across varying loads. It also offers redundancy in case one unit fails.

How does altitude and temperature affect generator sizing? Higher altitudes and temperatures reduce air density, affecting combustion and output. Always apply de-rating factors provided by manufacturers.

What’s the ideal load percentage for diesel generators? Between 70% and 80% of rated load is optimal for fuel efficiency and performance. Below 30% can lead to wet stacking.

Should I oversize my generator for future-proofing? Moderate oversizing (up to 20%) is reasonable if future expansion is likely. Avoid excessive oversizing that leads to inefficiency.

Conclusion

Diesel generator sizing isn’t just a technical step—it’s a strategic decision. 

Undersizing can cause operational chaos, while oversizing leads to hidden inefficiencies and long-term expenses. 

The smartest move is to size your generator precisely based on current and future power needs, using a combination of technical analysis and expert consultation.

If you’re planning a generator purchase or upgrade, take the time to analyze your load profile, understand your application, and work with professionals. 

With the right size diesel generator, you’ll save fuel, reduce maintenance, and ensure uninterrupted power for years to come.

Need Help Choosing the Right Diesel Generator?

Contact us for a free generator sizing consultation tailored to your specific application. Don’t gamble with power—let’s size it right from the start.

Explore more on:

• How to Maintain Your Diesel Generator for Maximum Lifespan

• Common Mistakes in Generator Installation and How to Avoid Them

• Top Diesel Generator Brands You Can Trust

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