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Factories depend on dependable electricity to keep production lines, motors, pumps, compressors, lighting and control systems operating. During a grid outage, a diesel generator can maintain essential operations, but selecting the right generator capacity requires more than adding the ratings of connected equipment.
An appropriately sized generator should match the factory's actual operating demand, account for starting loads and allow reasonable scope for future expansion. For facilities operating diesel generators for extended periods, RECD for DG Set considerations can also become relevant when evaluating the generator's emission-control requirements.
Begin With a Detailed Factory Load Assessment
The first step is to identify all electrical loads connected to the backup system. A factory may have production machinery, HVAC equipment, pumps, compressors, lighting, office systems and safety equipment.
However, every machine may not need to operate simultaneously during a power failure. Dividing loads into essential, priority and non-essential categories can provide a much more realistic picture of the required generator capacity.
Actual operating data is often more useful than simply relying on nameplate ratings. Electricity bills, energy meters, generator controllers and production records can help establish the factory's typical and peak demand.
For a more detailed look at this subject, the Aceget guide on generator sizing for factories explains the key considerations involved in matching generator capacity with industrial electrical demand.
Running Load and Starting Load Are Different
One of the most important considerations is the difference between running demand and starting demand.
Large motors used for compressors, pumps, conveyors and production machinery can draw considerably more current during startup. If several high-power motors start together, the temporary demand can cause a significant voltage drop if the generator is not properly selected.
A generator should therefore be evaluated for:
Continuous running load
Motor-starting requirements
Starting sequence
Power factor
Load diversity
Short-duration peak demand
Starting methods such as soft starters and variable-frequency drives can sometimes reduce the impact of motor starting, but their actual effect should be considered as part of the electrical design rather than assumed automatically.
Avoid Both Oversizing and Undersizing
An undersized generator can struggle with overloads, voltage instability and frequent protection trips. It may also fail to support important machinery when several loads operate simultaneously.
Oversizing creates a different problem. A large generator that spends most of its operating hours at very light load may not be an efficient choice for the application. Long periods of low-load operation can also create engine-operating concerns, particularly for diesel engines.
The ideal selection should therefore reflect the factory's real operating profile rather than simply adding a large safety margin.
BIS maintains standards relating to reciprocating internal-combustion engine driven generating sets. Its standards database references IS/ISO 8528, including requirements covering emergency power supply to safety services. The official BIS standards database can be used to review applicable Indian Standards.
Consider Future Factory Expansion
Generator selection should not only reflect today's production requirements. Factories frequently add machinery, production lines, chillers or additional shifts.
If the generator is selected with no consideration for future demand, the facility may need another major investment sooner than expected. At the same time, installing substantially more capacity than the foreseeable requirement can result in prolonged low-load operation.
A practical approach is to estimate present demand and then identify realistic expansion stages. This makes it easier to decide whether one larger generator, multiple smaller generators or a parallel-generator arrangement is more appropriate.
Multiple Generators Can Improve Flexibility
Large factories may benefit from using multiple generator sets instead of relying on a single very large unit.
For example, several generators operating in parallel can allow the facility to match running capacity with changing production demand. During periods of lower demand, fewer sets can operate. When production increases, additional sets can be brought online.
This arrangement can also provide redundancy. If one generator is unavailable, other units may continue supporting critical loads, depending on the system design.
Synchronization, load sharing, protection and changeover equipment must be engineered correctly for such configurations.
The Central Electricity Authority publishes official regulations and notifications concerning electrical safety and supply. Its CEA safety and electric-supply regulations page provides access to the applicable regulatory framework, including the 2023 regulations and subsequent amendments.
Installation Conditions Affect Generator Performance
The generator's physical environment is another important part of sizing and selection.
Factories may install DG sets in dedicated rooms, outdoor enclosures or designated utility areas. Ventilation, cooling-air movement, exhaust routing, fuel arrangements and maintenance access should all be considered.
Exhaust-system design is particularly important because the engine needs appropriate backpressure conditions and adequate removal of hot exhaust gases.
CPCB's official environmental standards documentation includes provisions concerning generator-set stack height and emission-related requirements. Its Environmental Regulation on Engines for Generator Set Application and Generator Sets provides an authoritative reference for generator-set emission and noise requirements.
Emission Compliance Should Be Considered Alongside Capacity
Factory generators can operate for significant periods, particularly where the utility supply is unreliable or production cannot tolerate interruptions. Consequently, environmental performance should be considered alongside electrical capacity.
CPCB's current documentation for new generator-set engines up to 800 kW gross mechanical power covers emission limits, certification procedures, testing requirements and NOx and particulate-matter control measures. The CPCB genset emission compliance document provides the official technical framework.
For an existing diesel generator, this is where RECD for DG Set planning can become relevant. Before selecting an emission-control retrofit, the generator's engine rating, exhaust flow, operating load, duty cycle, exhaust temperature and backpressure conditions should be evaluated. The emission-control solution should be compatible with the actual generator rather than selected solely on the basis of kVA.
Test the Generator Under Realistic Conditions
Commissioning and periodic testing are essential after installation. A factory should verify generator loading, voltage and frequency stability, automatic changeover, protection functions and response to major loads.
Load-bank testing can also provide useful information when actual production conditions cannot be reproduced during a scheduled test.
Keeping accurate generator load records is valuable because future sizing, maintenance and emission-control decisions can be based on actual operating data rather than assumptions.
A Practical Factory Generator Sizing Checklist
Before finalizing a generator, the facility should review:
Current maximum and average electrical demand.
Critical and non-critical production loads.
Motor-starting requirements.
Power factor and load diversity.
Expected future expansion.
Single-generator versus multiple-generator configuration.
Fuel storage and expected operating duration.
Ventilation and exhaust requirements.
Applicable electrical and environmental requirements.
Generator emission-control requirements where applicable.
The objective is not simply to install the biggest generator possible. A well-sized system should provide sufficient capacity, operate efficiently under the factory's real load profile and remain adaptable as production requirements change.
When an existing diesel generator is retained, electrical performance and environmental compliance should be considered together. Proper RECD for DG Set planning can complement generator sizing by ensuring that emission-control requirements are evaluated according to the generator's actual technical and operating characteristics.