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

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

First of all you need some head room to not only allow for future expansion and therefore increase in your peak demand but also because a generator runs best at 80% of its capacity rather than constantly at full load. On top of this, it is 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. For generators this typically varies between 55 and 65%. This means that a 100kVA generator can only take an immediate load of 55-65kVA 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 in to the generator size.

To avoid unnecessary oversizing of the generator, it is possible to configure the switchgear with a delay so that the circuits come online in a staggered fashion. This costs more on the changeover front but can save money on the generator and avoid a generator that ends up not having much load on (which in the long run can lead to issues such as wet stacking if it is not load banked).

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