Standby or prime: sizing a diesel generator for your site

Two generators with the same headline kVA figure can behave very differently depending on whether that figure is a standby rating or a prime rating. Knowing which one applies to your site is the first step to choosing the right set.

What the ratings mean

A standby rating is for emergency use during a mains failure, with a limited number of running hours each year. A prime rating is for sites that use the generator as a main source of power for long periods, for example where the grid is unreliable or unavailable.

Work out the load

• List every item the generator must run and its starting and running load
• Allow for motors, which draw several times their running current when they start
• Add a margin for future equipment rather than sizing exactly to today's load
• Confirm the supply: phases, voltage and frequency

An oversized generator running lightly loaded is as much a problem as an undersized one.

Do not forget the installation

Ventilation, exhaust routing, fuel storage, noise limits and the changeover panel all affect the final result. A sound-attenuated canopy, a bunded fuel tank and a properly rated changeover switch should be decided together with the generator, not added afterwards.