Single versus Parallel Generators
Single versus Parallel Generators (on photo: Standby power system with three Cummins 1400 kVA generator sets; credit:

7 factors to consider when deciding //

Paralleling is the synchronous operation of two or more generator sets connected together on a common bus in order to provide power to common loads as shown in Figure 1.

In deciding whether a single or multiple generators should be installed there are various factors to be considered, such as:

  1. Reliability
  2. Performance
  3. Cost
  4. Generator and Room size
  5. Efficiency
  6. Load variation
  7. Flexibility

1. Reliability

Reliability is the primary factor in the decision to use of paralleling in most emergency/standby applications, such as hospitals, computer centers and pumping stations; where the reliability of Power Supply is important since the loads connected are critical.

In these cases, use of multiple generator sets and prioritized loading of the system allows the more critical loads to be served at the expense of less critical loads.

Parallel generators with ATS
Figure 1 – Parallel generators with ATS (credit: Emerson)

In systems where all the loads are required for proper operation – Redundant generator sets are provided, so that failure of a generator set will not disable the facility.

Paralleling normally requires the ability to sequence loads in steps, and the ability to shed loads to allow the generator sets to operate within their load ratings in event of generator failure.

A multiple set installation should be sized to allow a generator set to be taken out of the system for routine maintenance or repair without jeopardizing the supply to the load.

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2. Performance

Performance of the on site power system can be more like the utility service when generators are paralleled, because the capacity of the aggregated generator sets relative to individual loads is much greater than it would be with single generator sets serving separate loads.

Because the bus capacity is greater, the impact of the transient loads applied to the generator sets by individual loads is minimized.

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3. Cost

In general, multiple paralleled generator sets will cost more than a single genset of the same capacity, unless the capacity requirement forces the design to machines operating at less than 1500 rpm.

The cost of a system should be evaluated as the total cost of ownership and must take into account factors such as:

  • The available building space,
  • Additional flues and pipe work,
  • Layout of cables,
  • Switchgear requirements and
  • A system control for multiple installations.

The required reliability and the benefit this brings must be set against the increased cost.

Cost of maintenance is a key factor with generator sets that run for prime power or co–generation schemes. Although a single large set may have a seemingly high capital cost, this may be mitigated by other factors associated with the installation costs of a multiple generator system.

NOTE // When evaluating total cost of ownership, the criticality of the installation will impact on the decision on the degree of redundancy that is built in to the system. Some local codes and standards require continuous service to legally required loads and the critical nature of some facilities may require similar service provisions.

If generator sets are paralleled, the maintenance cost and temporary down time associated with temporary generator sets can be avoided. These considerations may also impact on the number of sets required for the installation.

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4. Generator and room size

Generator and room size can be critical factors and may force a decision toward single or multiple set installations. A single generator set will typically be considerably heavier than a corresponding machine used in a paralleling situation.

Generator room
Generator room (photo credit:

For roof–top installations or where the set has to be maneuvered into a basement or other confined space this may be prohibitive, leading to a decision toward smaller, lighter generators.

However, space for access and maintenance must be allowed between the machines of a multiple installation and these inevitably use more room volume per electrical kilowatt generated.

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5. Efficiency

Efficiency is a vital factor if the power generation scheme is producing base load power or if is being used for tariff reduction or co–generation.

The versatility of the paralleling system, enabling generator sets to be run at optimal load and maximum efficiency will often pay back the initially higher installation costs in a short time in prime power situations.

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6. Load variation

Load is critical in deciding on the type of installation required. A single generator will typically be the most economical choice for loads below approximately 2000 kW as the cost of the paralleling control and switching equipment will be significant when compared
to the cost of the generator.

For small but essential installations, where the protection of two generators is essential but the cost of the paralleling equipment is prohibitive. A mutual standby installation may be a good alternative, where one generator acts as standby to the other.

For larger loads, the choice is less straightforward and around 2–3 MW, solutions using single or multiple generator sets are available. Above 3 MW, the choice is almost always multiple generator installations.

Note //

Although at first sight more economical, a single generator solution is also the least versatile and may be less cost–effective, particularly at partial loads and in long operating hour installations. In prime power applications, high speed diesel generator sets may provide lower overall life cycle cost, due to higher efficiency and lower maintenance cost than larger lower speed machines.

Load variation should be considered in any generator application decision as many applications exhibit large differences between day and night and between summer and winter load profiles.

A large manufacturing facility may have a daytime load of 2–3MW; but at night, unless used for continuous process application, the load may fall to just a few hundred kW or even less. Installing a single large generator into this application could lead to many hours of light load running, which is detrimental to the engine.

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A typical installation of this type might use four – 1000 kW generators, with a 500 kW generator in a paralleling scheme, where the daytime load uses three of the four sets and where at night, only the smaller set is required to run. Transient loads have a large effect on the required size of a generator and it is important to take into account all combinations of transient and steady state loads in any calculation to ensure that the power quality is maintained.

Note that some loads present leading power factor load to generator sets, and this is also required to be considered in the generator set sizing and sequence of operation for the system.

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7. Flexibility

Flexibility may be an important consideration where an installation may change in future.

A single generator set installation is usually difficult to change, whereas sets can be added to a multiple set installation with relative ease, provided that allowance has been made in the initial design.

This training video displays how a paralleling solution is set up and configured with the Atlas Copco Qc4002 controller and five QAS generators.

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Risks //

There are risks associated with the parallel operation of generator sets; both between sets and with the utility supply and these risks should be balanced with the benefits.

The risks are:


Where adequate load shedding has not been provided or where the load is maintained at a high level, there is a risk that, if one generator fails, the remaining generators on the system may not be able to support the system load.

Load shedding should always be incorporated into a paralleling generation scheme and the reserve capacity at any time during operation should correspond with the amount of load that can be accepted if a running generator fails.


Not all generators can be paralleled together – if sets of a different manufacturer or of a significantly different size are to be paralleled, consult the local Cummins distributor before proceeding.


When paralleling with the utility, the generator effectively becomes a part of the utility system. If operation in parallel with the utility supply is specified, additional protection is required for the protection of the generator and utility system interconnection. This protection is commonly specified and approved by the utility service provider.

Always consult local codes and standards when considering utility parallel operation.

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Reference // Application Manual – Liquid Cooled Generator Sets (Chapter 5 – Electrical Design) – Cummins Power Generation

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. engr.ajmal
    Sep 19, 2015

    deer sir
    I am working on designing a power plant of 5MW using the 5 gen set of 1MW please guide me how i can design them and synchronizing them for load sharing using DEEP SEA control module. and if you have notes please share with me.



  2. saad rehman
    Aug 14, 2015

    deer sir i.m working on two engine type duets 1mva with woodward speed control to operate theme in parallel can you sent your e mail to sent the synchronizer and load shearing device type for your advice

  3. Edward Werderits
    Jun 15, 2015

    I’m having a problem with the sine wave synchronization, how does the output of multiple gen look on a scope ?

  4. st01
    May 09, 2015

    great article big thanks

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