Why do I need a larger generator than my peak demand?

When sizing a generator, it is easy to conclude that if my peak demand power usage is 100kVA that I can get away with a 100kVA generator.
However, that’s not the case.

First of all you need some head room to allow for spike loads as well as  allowing for future expansion because over time your power demand will  increase.

Secondly, all generators have a power factor of 0.8.

On top of this, it’s important to consider the acceptance load figures for generators. This is the initial amount of load (as a percentage of the peak load) that the generator is able to accept immediately upon demand.

Peak Demand Load Example

For all generators this typically is 65% of the capacity of the generator.

As an example, this means that a 100kVA generator can only take an immediate load of 65kVA without falling over. The other consideration is that certain equipment (e.g. motors) can have a starting current that is multiple times more than their normal running current. You need to factor this into the generator size.

Aerospace Generator Peak Demand

Motor drives, pumps, print machines, Air Con units all have a power surge upon start up.
These can spike as high as 7 times  higher than their continuous running power demand.
Just confirming the regular power usage is deceptive as it doesn’t clearly explain the start up power spike.
That power spike can overload the diesel generator, causing it to fail.

Therefore its prudent to calculate all the site loads and be sure to include all the site peak demand loads before installing a diesel generator.

Automatic Mains Fail Transfer Panel ( AMF )

An automatic mains fail transfer panel (AMF) will control the start and stop of your generator power system.

Should there be many heavy machinery loads that require emergency power, then these loads can be placed, in a controlled manner, onto the generator once it has started

This is handled by multiple AMF panels, configured with time delays.
These circuits will come online as step loads.
For example, every 30 seconds another load is added to the generator.

We would advise not running any generator continually with move than 80% of its capacity. This 20% headroom allows for any unknown spike load as well as any unknown new equipment that may have been installed at site without knowledge.

A Generator is for life, not just Christmas.

With careful design planning you’ll have a fully installed automatic emergency power system, that will last for 30 years of faithful service.

Always remember to service and maintain your diesel generator as well as regular on building load tests.


Once a year, establish a recoded regime to:

  • Carry out a load bank test
  • Fuel Polishing to keep your fuel uncontaminated
  • Carry staff training to keep up to date with the site developments
  • Check the control panel software for any new updates
  • Record the maximum power demand spike during the last 12 months and record it.
  • Check earthing is intact
  • Check the weekly OFF load tests have been recorded accurately
  • Ensure the monthly ON load tests have been carried out and recorded

For more information contact Power Continuity Engineers on 0845 055 8455 


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