Choosing a diesel generator should never begin with a random kVA number. Before selecting a generator, the facility needs to understand how much electrical power its equipment actually requires, which loads operate together, and which machines create high starting demand.
Accurate load calculation helps avoid two common problems: an undersized generator that struggles during peak demand and an oversized generator that operates inefficiently at light loads. The same information can later be useful when evaluating the operating profile of a diesel generator and planning RECD for DG Set requirements.
Start by Listing Every Electrical Load
The first step is to create a complete inventory of equipment that may need backup power.
Depending on the application, the list could include:
Lighting systems
Fans and exhaust systems
Air conditioners
Refrigerators and freezers
Computers and networking equipment
Water pumps
Compressors
Industrial motors
Conveyors and machinery
Elevators
Security and surveillance equipment
The important point is completeness. Leaving a significant motor, pump or cooling system out of the calculation can result in a generator that looks adequate on paper but fails when the equipment is actually connected.
Loads should also be separated into continuous and intermittent categories. Lighting and computers may operate continuously, while pumps, compressors and air-conditioning systems may cycle on and off.
For a practical explanation of this calculation process, the Aceget guide on load calculation before buying a generator walks through load inventory, nameplate readings, starting demand, power factor and generator selection.
Understand Watts, kW, VA and kVA
Generator sizing becomes easier when the basic electrical terms are understood correctly.
Watts and kW represent real power—the power actually used to perform work. VA and kVA represent apparent power, which also accounts for the electrical current associated with reactive components.
The relationship is influenced by power factor:
kW = kVA × Power Factor
Therefore:
kVA = kW ÷ Power Factor
For example, a 6 kW requirement at a power factor of 0.8 corresponds to 7.5 kVA.
This distinction is important because generator sets are commonly specified by their kVA rating. Using kW and kVA interchangeably can lead to an incorrect generator selection.
Read Equipment Nameplates Carefully
Equipment nameplates are among the most useful sources of information when calculating load.
Depending on the equipment, a nameplate may show voltage, current, wattage, kW, power factor or motor information. For motor-driven equipment, starting current or locked-rotor information can also be important.
The calculation should use actual equipment data wherever possible instead of relying on approximate assumptions. This becomes particularly important in factories, workshops and commercial facilities containing several motors.
For installations involving generator sets, applicable technical standards should also be considered. The Bureau of Indian Standards (BIS) provides India's official standards framework, including standards relevant to electrical equipment and generating-set applications.
Running Load Is Not the Same as Starting Load
One of the biggest mistakes in generator sizing is considering only running watts.
Electric motors can draw significantly higher current for a short period when they start. Pumps, compressors, air-conditioning systems and industrial machinery can therefore create a substantial temporary demand.
Imagine a facility with a total running requirement of 8 kW. If a large compressor requires a considerable starting surge, an 8 kVA generator may not necessarily be sufficient even though its nominal capacity appears close to the running requirement.
Starting characteristics should therefore be evaluated alongside normal operating demand. Starting sequence can also matter. If multiple motors start simultaneously, the temporary demand can be considerably higher than when the same motors are started sequentially.
Apply Demand and Diversity Carefully
Not every connected device necessarily operates at full rated capacity at the same time.
Demand factor helps account for the difference between total connected load and actual maximum demand. Diversity becomes useful when different groups of loads have separate peak periods.
For a small office or shop, a straightforward load assessment may be sufficient. For a larger factory, hospital or industrial facility, engineers may need a more detailed demand study involving multiple departments, production lines and operating schedules.
The objective is not simply to reduce the calculated number. The calculation should represent realistic simultaneous demand without making optimistic assumptions that could compromise generator performance.
Add a Sensible Capacity Margin
After calculating the expected demand and starting requirements, a reasonable margin can provide protection against estimation errors and future load additions.
However, the margin should not become an excuse to oversize the generator dramatically. A generator that consistently operates at very low load can be an inefficient choice and may not match the engine's intended duty profile.
A better approach is to consider both foreseeable expansion and the generator's actual operating pattern before deciding the final rating.
Consider Generator Duty and Application
A generator used occasionally during utility failures may have different requirements from one operating for long periods as a primary power source.
Standby, prime and continuous applications have different operating expectations. Therefore, the final selection should consider not only the calculated kVA but also:
Expected operating hours
Average load
Peak load
Starting requirements
Ambient conditions
Fuel availability
Maintenance requirements
Future expansion
This ensures that the selected generator matches the application rather than simply matching one calculated number.
Load Calculation Also Supports Emission Planning
Load assessment has another useful application for existing diesel generators.
The generator's actual load profile can influence engine operating conditions, fuel consumption and exhaust characteristics. When an older DG set is being evaluated for emission-control requirements, understanding how it is operated provides useful technical context.
CPCB's official Environmental Regulation on Engines for Generator Set Application and Generator Sets covers generator-set emission and noise requirements and identifies different generator-set categories.
For newer internal-combustion engines used in genset applications, CPCB's official compliance documentation also covers emission limits, certification procedures, testing requirements and NOx/PM control measures. The CPCB genset emission compliance document provides the relevant technical framework.
Where an existing generator requires an emission-control retrofit, RECD for DG Set planning should therefore be based on the actual generator specifications and operating conditions rather than kVA alone.
Don't Forget Installation Conditions
Load calculation is only one part of generator selection. The installation environment also matters.
Ventilation, exhaust routing, fuel storage, cooling, electrical protection and maintenance access should be reviewed before finalizing the generator.
CPCB's official environmental standards documentation also addresses DG-set stack-height requirements and provides technical guidance based on generator capacity. The CPCB environmental standards document includes the DG-set stack-height provisions.
A Simple Load-Calculation Workflow
A practical calculation can follow these steps:
List every load requiring backup power.
Record actual nameplate ratings.
Separate continuous and intermittent loads.
Identify motors and other high-starting-current equipment.
Calculate realistic simultaneous running demand.
Account for starting requirements.
Convert the final requirement into kVA using appropriate power factor.
Add a sensible capacity margin.
Check the result against the generator's duty rating.
Review applicable electrical and emission requirements.
Final Thoughts
Load calculation is the foundation of sensible generator selection. A reliable result comes from understanding what equipment will operate, how much power it actually consumes and what happens when motor-driven loads start.
For an existing diesel generator, the same operational information can support broader maintenance and environmental decisions. When emission-control requirements apply, proper RECD for DG Set planning should consider the generator's engine, capacity, exhaust conditions and real operating profile.
The goal is not to choose the biggest generator available. It is to select a system with sufficient capacity, appropriate operating characteristics and a realistic margin for the application.