Choosing the right diesel generator size for a factory is not as simple as adding up the rated power of every machine. The generator must be large enough to supply the required loads, start motor-driven equipment, and handle normal load changes without unnecessary oversizing.

For factories considering a diesel generator for backup or prime power, the most important question is:

How much power does the factory actually need when the generator is running?

The answer depends on the factory's operating equipment, load profile, motor starting requirements, electrical system, and future expansion plans.

1. Start With the Loads That Must Be Powered

A factory may have dozens or hundreds of electrical devices, but not all of them need to operate when the utility power fails.

The first step is to divide the electrical loads into different categories.

These may include:

• Production machinery
  
• Air compressors
  
• Water pumps
  
• Fans and ventilation
  
• Refrigeration equipment
  
• HVAC systems
  
• Lighting
  
• Office equipment
  
• Security and monitoring systems
  
• Fire protection equipment
  
• Other critical loads
  

For a backup generator, the most important question is often not:

“What is the total installed capacity of the factory?”

but:

“Which loads must continue operating during a power outage?”

Reducing non-essential loads can significantly change the required generator size.

2. Check the Actual Running Load

Equipment nameplate ratings do not always represent actual operating consumption.

A machine rated at 20kW may not continuously consume 20kW. Its real power demand can vary depending on production conditions.

This is why factories should ideally collect actual load data through their electrical distribution system or power monitoring equipment.

Useful information includes:

• Minimum load
  
• Normal load
  
• Peak load
  
• Maximum simultaneous load
  
• Load fluctuations
  

A generator should be selected according to realistic operating conditions rather than theoretical maximum capacity alone.

3. Motor Starting Can Be the Critical Factor

Many factories use motors for compressors, pumps, fans, conveyors, machine tools, and production equipment.

The power required during motor startup can be significantly higher than the motor's normal running demand.

This means a factory may appear to need only 150kW during normal operation but still require a larger generator because a large motor needs to start while the generator is already supplying other loads.

For this reason, the following information should be collected for large motors:

Motor rated power

Starting method

Starting current

Starting sequence

Whether multiple motors start at the same time

Direct-on-line starting can create a larger starting impact than some controlled starting methods.

Soft starters and variable frequency drives can reduce starting impact and may help optimize generator sizing.

4. Power Factor Also Matters

Generators are commonly specified in both kW and kVA.

kW represents real power, while kVA represents apparent power.

The relationship between them is affected by the power factor.

For example:

kW = kVA × Power Factor

If a factory has many inductive loads such as motors, transformers, and compressors, the power factor may have a meaningful impact on generator sizing.

Therefore, buyers should not simply compare the generator's kW rating with the factory's total electrical load without considering the complete electrical characteristics.

5. How Do You Estimate Generator Capacity?

A simple first estimate can begin with the maximum expected simultaneous running load.

For example, suppose the critical factory loads are:

• Production equipment: 90kW
  
• Pumps: 20kW
  
• Compressors: 30kW
  
• Lighting: 10kW
  
• Ventilation: 10kW
  

The estimated running load is:

160kW

However, this does not automatically mean that a 160kW generator is the correct choice.

The buyer should still check:

• Motor starting requirements
  
• Power factor
  
• Load fluctuations
  
• Required reserve capacity
  
• Future expansion
  
• Prime or standby operation
  

The final generator size should therefore be confirmed using the actual load profile and the generator manufacturer's technical recommendations.

6. Should You Choose a Generator Larger Than the Load?

Some reserve capacity is useful, but excessive oversizing is not always beneficial.

A generator that is too small may experience overload, voltage dips, difficulty starting motors, or unstable operation.

A generator that is unnecessarily large may operate at low load for extended periods, which can reduce operating efficiency and may not be ideal for the engine.

The goal is to find a practical balance:

Enough capacity for the real application + sufficient allowance for starting and reasonable future growth.

This is more useful than simply choosing the largest generator available.

7. Example: Selecting a Generator for a Small Factory

Consider a factory with a normal critical load of approximately 220kW.

Its main electrical equipment includes:

• Two 55kW motors
  
• One 45kW air compressor
  
• Pumps totaling 30kW
  
• Lighting and control systems totaling 15kW
  
• Other essential equipment totaling 20kW
  

The normal critical load is approximately:

220kW

However, the two 55kW motors may create significant starting demand.

The factory therefore should not simply order a 220kW generator without checking the motor starting characteristics.

The supplier may recommend a larger generator, a different alternator configuration, or a controlled motor-starting strategy depending on the operating sequence.

This example shows why generator sizing must consider more than the arithmetic sum of running loads.

8. Standby or Prime Power?

The required generator size also depends on how the factory intends to use it.

Standby Power

If the generator is only used during utility failures, the project may be designed around the critical emergency loads.

The generator should be capable of supplying those loads reliably when the grid is unavailable.

Prime Power

If the generator will operate for long periods as the main source of electricity, the load profile and operating hours become even more important.

Fuel efficiency, engine selection, maintenance intervals, cooling performance, and continuous operating capability all become important considerations.

The same factory may therefore require a different generator configuration depending on whether the unit is intended for standby or prime power.

9. Do You Need to Power the Entire Factory?

Not necessarily.

Many factories can reduce the required generator capacity by separating essential and non-essential loads.

For example:

Essential Loads

• Critical production machinery
  
• Safety systems
  
• Fire pumps
  
• Emergency lighting
  
• Security systems
  
• Essential ventilation
  

Non-Essential Loads

• Office air conditioning
  
• Non-critical production equipment
  
• Decorative lighting
  
• Optional machinery
  
• Other loads that can remain offline
  

A properly designed emergency power system can prioritize the essential loads first.

This may allow the factory to use a smaller and more economical generator while maintaining critical operations.

10. Automatic Transfer Is Important for Factory Backup Power

For factories where production must resume quickly after a utility failure, an automatic transfer system can be important.

A typical sequence is:

Utility power failure

↓

Generator starts automatically

↓

Generator reaches stable voltage and frequency

↓

Load transfers to generator

↓

Generator supplies critical loads

When utility power returns, the system can transfer the load back and shut down the generator according to the configured control logic.

An Automatic Transfer Switch, or ATS, is therefore often included in factory backup power systems.

Whether ATS is required should be confirmed during project planning rather than treated as an afterthought.

11. Consider the Factory's Electrical Environment

Generator sizing also needs to take the factory's electrical infrastructure into account.

Important factors may include:

• Main distribution voltage
  
• Frequency
  
• Transformer capacity
  
• Existing switchgear
  
• Motor control systems
  
• Automatic transfer equipment
  
• Cable capacity
  
• Protection system
  
• Generator connection point
  

The generator should be compatible with the existing electrical distribution system.

A correct kW rating alone does not guarantee that the generator can be integrated safely and effectively into the factory's power system.

12. Think About Future Expansion

Factories often add new machinery as production capacity grows.

If the current critical load is 180kW but a major production expansion is planned, the generator selection should take the future project into account.

However, the future allowance should still be realistic.

Building excessive generator capacity today for an expansion that may never happen can increase the initial investment and create low-load operating conditions.

A better approach is to discuss the realistic expansion plan with the generator supplier and determine whether:

• A larger single generator is appropriate
  
• Additional generators can be added later
  
• Parallel operation should be considered
  
• The electrical system should reserve space for future equipment
  

13. When Should You Consider Multiple Generators?

For larger factories or facilities with highly variable loads, multiple generator sets can sometimes be more flexible than one very large generator.

For example, instead of using one large generator for every situation, a facility may operate:

• One generator during light loads
  
• Two generators during medium loads
  
• Additional units during peak demand
  

This approach can improve flexibility and provide redundancy.

However, it also requires synchronization, load sharing, control equipment, and appropriate electrical protection.

The decision should be based on the site's load profile, reliability requirements, budget, and operational strategy.

14. What Information Should a Factory Provide to a Generator Supplier?

The fastest way to get a useful recommendation is to provide detailed technical information.

At minimum, include:

Factory application

Required backup loads

Maximum and normal load

Large motor sizes

Motor starting methods

Voltage

Frequency

Standby or prime power

Indoor or outdoor installation

Required operating time

Noise requirements

Future expansion plans

A recent electricity bill or load-monitoring report can also be helpful where available.

With this information, the supplier can evaluate the load profile and recommend a more suitable generator capacity.

15. Common Mistakes in Factory Generator Sizing

Several mistakes occur frequently during generator selection.

Mistake 1: Adding Every Nameplate Rating

This can result in unnecessary oversizing because not all machines operate simultaneously.

Mistake 2: Ignoring Motor Starting

A generator may handle the running load but fail to start a large motor.

Mistake 3: Choosing the Largest Available Generator

Larger capacity is not automatically better and may lead to inefficient low-load operation.

Mistake 4: Ignoring Voltage and Frequency

A generator must match the factory's electrical system.

Mistake 5: Forgetting Future Changes

If a realistic production expansion is planned, it should be considered before the generator is purchased.

Mistake 6: Comparing Only the Generator Price

The complete system may also include ATS, switchgear, cabling, installation, commissioning, and other equipment.

16. How COMAN POWER Can Help With Factory Generator Selection

COMAN POWER provides diesel generator sets for industrial and commercial applications and offers generator configurations across different power ranges.

For factory projects, the recommended generator configuration can be determined according to the required load, operating mode, electrical specifications, installation environment, and additional control requirements.

Depending on the project, the solution can also include options such as silent enclosures, automatic control, ATS, remote monitoring, or other customized configurations.

The objective is not simply to supply a generator with a particular kW rating, but to provide a generator set that matches the factory's actual operating conditions.

17. Final Checklist Before Ordering

Before purchasing a diesel generator for a factory, confirm the following:

1. What loads must remain powered?

2. What is the normal operating load?

3. What is the maximum simultaneous load?

4. Which motors have high starting requirements?

5. What starting methods are used?

6. What voltage and frequency are required?

7. Is the generator for standby or prime power?

8. Does the site need ATS or automatic operation?

9. What is the installation environment?

10. Is future expansion expected?

Once these questions have been answered, generator sizing becomes much more straightforward.

Choosing the Right Generator Size for a Factory

There is no universal generator size for every factory.

A 100kW generator may be suitable for one facility, while another factory may require 300kW, 500kW, or a much larger system.

The correct solution depends on the actual load profile, motor starting requirements, operating mode, electrical configuration, and future plans.

The best approach is to avoid both under-sizing and excessive oversizing.

Choose the generator based on the loads that actually need reliable power—not simply the total number of machines in the factory.