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Home / Technical Articles / How not to select the wrong generator set (genset) for your application

Generator Set Sizing and Ratings

Generator set is maybe the most expensive part you can find in a substation. Therefore, many factors must be considered when determining the proper size or electrical rating of an electrical power generator set. If wrong size or rate is selected and delivered on site, the only thing you can do is to sit and cry.

How not to select the wrong generator set (genset) for your application
How not to select the wrong generator set (genset) for your application (photo credit: beyond the prism photography via Flickr)

The engine or prime mover is sized to provide the actual or real power in kW, as well as speed (frequency) control through the use of an engine governor.

The generator is sized to supply the kVA needed at startup and during normal running operation. It also provides voltage control through the use of a brushless exciter and voltage regulator. Together the engine and generator provide the energy necessary to supply electrical loads in many different applications encountered in today’s society.

The generator set must be able to supply the starting and running electrical load. It must be able to pick up and start all motor loads and low power factor loads, and recover without excessive voltage dip or extended recovery time.

Nonlinear loads like variable frequency drives, uninterruptible power supply (UPS) systems and switching power supplies also require attention because the SCR switching causes voltage and current waveform distortion and harmonics. The harmonics generate additional heat in the generator windings, and the generator may need to be upsized to accommodate this.

The type of fuel (diesel, natural gas, propane, etc.) used is important as it is a factor in determining generator set response to transient overloads.

It is also necessary to determine the load factor or average power consumption of the generator set. This is typically defined as the load (kW) x time (hrs. while under that particular load) / total running time. When this load factor or average power is taken into consideration with peak demand requirements and the other operating parameters mentioned above, the overall electrical rating of the genset can be determined.

Other items to consider include the unique installation, ambient, and site requirements of the project. These will help to determine the physical
configuration of the overall system.

Typical rating definitions for diesel gensets are: standby, prime plus 10, continuous and load management (paralleled with or isolated from utility).

Any diesel genset can have several electrical ratings depending on the number of hours of operation per year and the ratio of electrical load/genset rating when in operation.

The same diesel genset can have a standby rating of 2000 kW at 0.8 power factor (pf) and a continuous rating of 1825 kW at 0.8 pf. The lower continuous rating is due to the additional hours of operation and higher load that the continuous genset must carry. These additional requirements put more stress on the engine and generator and therefore the rating is decreased to maintain longevity of the equipment.

Different generator set manufacturers use basically the same diesel genset electrical rating definitions. These are based on International Diesel Fuel Stop Power standards from organizations like ISO, DIN and others.

Standby diesel genset rating

Typically defined as supplying varying electrical loads for the duration of a power outage with the load normally connected to utility, genset operating <100 hours per year and no overload capability.

Prime plus 10 rating

Typically defined as supplying varying electrical loads for the duration of a power outage with the load normally connected to utility, genset operating <500 hours per year and overload capability of 10% above its rating for 1 hour out of 12.

Continuous rating

Typically defined as supplying unvarying electrical loads (i.e., base loaded) for an unlimited time.

Load management ratings

Apply to gensets in parallel operation with the utility or isolated/islanded from utility and these ratings vary in usability from <200 hours per year to unlimited usage.

It’s highly advisable to refer to generator set manufacturers for further definitions on load management ratings, load factor or average power consumption, peak demand and how these ratings are typically applied.

Even though there is some standardization of these ratings across the manufacturers, there also exists some uniqueness with regard to how each manufacturer applies their generator sets.

Electrical rating definitions for natural gas powered gensets are typically defined as standby or continuous with definitions similar to those mentioned above for diesels.

Natural gas gensets recover more slowly than diesel gensets when subjected to block loads. Diesel engines have a much more direct path from the engine governor and fuel delivery system to the combustion chamber, resulting in a very responsive engine-generator.

A natural gas engine is challenged with air-fuel flow dynamics and a much more indirect path from the engine governor (throttle actuator) and fuel delivery system (natural gas pressure regulator, fuel valve and actuator, carburetor mixer, aftercooler, intake manifold) to the combustion chamber. This results in a less responsive engine-generator.

Diesel gensets recover about twice as fast as natural gas gensets!

Genset software

Considering factors such as site conditions, load characteristics and required performance, SpecSizer (Caterpillar’s genset software) provides accurate and timely technical data, robust load models and optimized algorithms to assist in specifying a properly sized generator set to best meet your power needs.

Several features set SpecSizer apart as a revolutionary tool in generator set sizing, including its ability to evaluate load types such as air conditioners, elevators, ultraviolet lights, single-phase NEMA and single-phase IEC motors.

For the actual calculations involved for sizing a genset, there are readily accessible computer software programs that are available on the genset manufacturer’s websites or from the manufacturer’s dealers or distributors. These programs are used to quickly and accurately size generator sets for their application.

The programs take into consideration the many different parameters discussed above, including:

  • The size and type of the electrical loads (resistive, inductive, SCR, etc.),
  • Reduced voltage soft starting devices (RVSS),
  • Motor types,
  • Voltage,
  • Fuel type,
  • Site conditions,
  • Ambient conditions and
  • Other variables.
The software will optimize the starting sequences of the motors for the least amount of voltage dip and determine the starting kVA needed from the genset. It also provides transient response data, including voltage dip magnitude and recovery duration. If the transient response is unacceptable, then design changes can be considered, including oversizing the generator to handle the additional kvar load, adding RVSS devices
to reduce the inrush current, improving system power factor and other methods.

The computer software programs are quite flexible in that they allow changes to the many different variables and parameters to achieve an optimum design. The software calculates how to minimize voltage dips and can recommend using paralleled gensets vs. a single genset.

Genset Sizing Guidelines

Some conservative rules of thumb for genset sizing include:

  1. Oversize genset 20–25% for reserve capacity and for across the line motor starting.
  2. Oversize gensets for unbalanced loading or low power factor running loads.
  3. Use 1/2 hp per kW for motor loads.
  4. For variable frequency drives, oversize the genset by at least 40% for sixpulse technology drives.
  5. For UPS systems, oversize the genset by 40% for 6 pulse and 15% for 6 pulse with input filters or 12 pulse.
  6. Always start the largest motor first when stepping loads.

For basic sizing of a generator system, the following example could be used:

Step 1 – Calculate Running Amperes

  • Motor loads:
    • 200 hp motor: 156 A
    • 100 hp motor: 78 A
    • 60 hp motor: 48 A
  • Lighting load: 68 A
  • Miscellaneous loads: 95 A
  • Running amperes: 445 A

Step 2 – Calculating Starting Amperes Using 1 25 Multiplier

  • Motor loads:
    • 200 hp motor: 195 A
    • 100 hp motor: 98 A
    • 60 hp motor: 60 A
  • Lighting load: 68 A
  • Miscellaneous loads: 95 A
  • Starting amperes: 516 A

Step 3 – Selecting kVA of Generator

  • Running kVA = (445 A × 480 V × 1.732) / 1000 = 370 kVA
  • Starting kVA = (516 A × 480 V × 1.732) / 1000 = 428 kVA


Generator must have a minimum starting capability of 428 kVA and minimum running capability of 370 kVA.

Typical genset installation
Figure 1 – Typical genset installation

Generator Set Installation and Site Considerations

There are many different installation parameters and site conditions that must be considered to have a successful generator set installation. The following is a partial list of areas to consider when conducting this design.

Some of these installation parameters include:

  1. Foundation type (crushed rock, concrete, dirt, wood, separate concrete inertia pad, etc.)
  2. Foundation to genset vibration dampening (spring type, cork and rubber, etc.)
  3. Noise attenuation (radiator fan mechanical noise, exhaust noise, air intake noise)
  4. Combustion and cooling air requirements
  5. Exhaust backpressure requirements
  6. Emissions permitting
  7. Delivery and rigging requirements
  8. Genset derating due to high altitudes or excessive ambient temperatures
  9. Hazardous waste considerations for fuel, antifreeze, engine oil
  10. Meeting local building and electrical codes
  11. Genset exposure (coastal conditions, dust, chemicals, etc.)
  12. Properly sized starting systems (compressed air, batteries and charger)
  13. Allowing adequate space for installation of the genset and for maintenance (i.e., air filter removal, oil changing, general genset inspection, etc…)
  14. Flex connections on all systems that are attached to the genset and a rigid structure (fuel piping, foundation vibration isolators, exhaust, air intake, control wiring, power cables, radiator flanges/duct work, etc.)
  15. Diesel fuel day tank systems (pumps, return piping)
  16. Fuel storage tank (double walled, fire codes) and other parameters

Please see the generator set manufacturer’s application and installation guidelines for proper application and operation of their equipment.

4x1225KW, 13.8KV Diesel Generator sets in Parallel

Source: Power Distribution Systems by Eaton

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. leenkw
    Nov 21, 2021

    In singapore ,there is a a reluctance to adopt MV generators as
    1, They are not cheap to operate than LV generators
    2, higher maintenance than LV generators, consuming more lube oil and fuel
    3. Min emergency load of 30% is higher than LV generators otherwise hunting will occur
    4, May have to adopt more than 1 MV Automatic transfer switch as it is limited in size

  2. demewoz
    Mar 05, 2021

    thank you for your practical and ideal information specially about power supply and distribution.

  3. sangkar subramaniam
    Feb 05, 2021

    How can you calculate gen set in Kva to AMPS, please explain.

  4. Graham Roy
    Feb 04, 2021

    A great introduction to the art of backup generation.

    Having been the designer of a few Turbine generator control systems and Power Management Systems I know how hard it can be to get the concepts over. Throwing a large motor (comparative to the genset) on to the bars can always be exciting. It is vitally important to ensure the load acceptance and rejection properties of the whole of the system is known.

    There many other items to consider e.g. the speed governing mode of the sets; isochronous or droop, the overall governing mode e.g. kW follower, temperature control, frequency control, or maybe the prime driver type, rotating machine or reciprocating machine. If in doubt your best bet is to ask an specialist for advice.

  5. Peter Maes
    Feb 04, 2021

    More and more sites in Europe ( e.g Ireland) request the need that plants are able to run daily 2-3 hours independantly from the power grid as there is insufficient generating capacity at peak hours. So many datacenters are equipped with there own genset systems. Though for this type of applications diesel genset are no longer suitable, as they do not comply with the more stringent emmission norms. -> So there is a shift in Europe in the use of Gas Generating sets (reciprocating). Todays Gas engines are as well suitable for fast start-up applicationss. The only thing you need is a gas grid interconnection (gas grid has a high reliability here in Europe)

  6. Teboho Noko
    Aug 05, 2019

    We are a very poor and undeveloped country, we are going to open a small plant and run it using generators full time and with this generator we will also be running everything(cooking, lighting, e.t.e…..) with it. Now want to know how can calculate the size of generator that we will need.
    Can you help please


  7. Prashantha
    Aug 02, 2019

    Very very good and valuable information.Thank you for sharing such valuable ideas.

  8. A K BAKSI
    Aug 01, 2019

    While selecting a DG set , pl hilight which one is better between INLINE ENGINE OR V-TYPE ENGINE for performane and maintenance point of view .

  9. Selvan
    Aug 01, 2019

    How to share your valuable thoughts??

  10. Robinson. S
    Aug 01, 2019

    If motor work synchronous speed, if doesn’t conduct synchronous speed at motor as like a generator and then check voltage in particular generator capacity

  11. Faraz Ahmed Naeem
    Jul 31, 2019

    Very informative article Edvard.

    How can you calculate tha 200 hp motor load, please explain.


  12. Maxime abdi aynab
    Jul 31, 2019

    Thank you very much.
    You have given a refreshing knowledge and it was very interesting.

    Jul 31, 2019

    High Impedance Fault

  14. Pratap
    Jul 31, 2019

    Very well explained. Shoul also explain about precautions and selection criteria for paralleling two or three sets.
    Shoul explain about PF and effect on rating or tested power.

  15. Belletto
    Jul 31, 2019

    Very synthetic and useful description. Thank you for sharing.

    Just two additional concerns I would mention :

    1- be careful when the majority of loads exhibit leading power factor (like IT equipment): the capability for the alternator to import reactive power produced by this equipment is very limited, much lower than his capacity to export it, for example, to motors.

    2- for fault current calculation, be aware that alternators have a zero-sequence fault current level higher than the bolted fault current level.

    • Abbas Ahmad
      Oct 17, 2020

      I want job Diesel engine mechanic good experience 17years

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