Sizing a diesel generator

Sizing a diesel generator for the job can be a difficult task and isn’t just dependent upon the job at hand.

It’s easy to calculate the power output required for general household appliances with a simple addition of kVA, ampere or KW. However, this is a common misconception for calculating the load requirements for all pieces of equipment.

Things become tricky when you need to size a diesel genset for pieces of equipment that require high initial startup loads – a common requirement of equipment that produce high amounts of pressure like water pumps. Many people think that they can purchase a generator based on the calculation just by adding the kVA, KW or ampere, but this is not the case.

It can be difficult to determine what size genset you need for certain pieces of equipment, and we have heard of many customer experiences where they have purchased a genset only to find out later that they haven’t taken into consideration the high initial startup load and low ongoing load compared to the startup load.

It’s critical to correctly size your genset. If the generator is too small for the load, then it will cause voltage fluctuations and permanent damage to the equipment. If the generator is too large for the load, then it will cause the engine to glaze over, and carbon build up.

General Rule of Thumb when sizing a diesel generator

  • Size the generator for your equipment to run between 60 – 80% load capacity of the diesel generator.
  • Warm up the generator before applying the load – Approximately 5 – 10 minutes is a sufficient warm up time
  • Have an electrician balance the load across all phases for single phase loads.

Understanding the difference between initial startup load and ongoing load requirements

The startup load of any genset can be two or three times higher than the ongoing load. The key components of any genset to consider when sizing for high initial startup loads are:

  • The type of engine starter used
  • How the genset is wired.

The three most common engine starters used in diesel gensets are:

  1. Star Delta (S.D) - Smooth start up, medium – to – high current
  2. Direct On Line (D.O.L)- High Startup current (A)
  3. Soft Start- Controls motor’s voltage supply during start – up phase

The table below demonstrates the differences in the load capacity for gensets based on the type of engine starter. We have used water pumps as the piece of equipment to show the differences between them.

1 hp equals to 0.75KW, 1kVA equals to 0.8KW (5hp x 0.75= 3.75KW) (5kVA x 0.8= 4KW)

Pump Size Generator Requirement (kVA) for Startup Ongoing Load
HP KW D.O.L to kVA S.D to kVA kVA During Ongoing Load
1 0.75 2.5 2 1
1.5 1.1 3.75 3 1.5
2 1.5 5 4 2
3 2.2 7.5 6 3
4 3 10 8 4
5 3.7 12.5 10 5


Gogopower provides this calculated table as a guide only to assist with general queries and recommends you work with experts to determine the most suitable diesel generator for your needs.

Gogopower assumes no responsibility or liability for any errors or misuse from the content of this article. This article does not constitute as experts’ advice.

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