Rotary UPS really make most sense if you want 'cost no option' uninterrupted power, as even when not used, there is a lot of energy and maintenance require to keep tons of metal spinning on the off chance there is a power cut.

traditional gensets take time (seconds to tens of seconds) to start, perhaps longer if they need to be synchronized with other sources, and if not used for a long time occasionally do not start, so while less effort do need a regime of periodic testing, including things like is the fuel still OK, is the cranking battery holding charge etc.

Static UPS (a big inverter) can give glitch free handover,  and may be undersized a bit if used either just to cover genset spin-up time, or larger if to be used for longer periods with very large batteries.  Attractive though htis sounds, batteries do not last for ever and are considerable capex and space.

It is important to know how much and what loads are to be shed totally (try and maximise this), interrupted briefly (many things), or maintained without glitch (try and minimise this). This load distribution drives exactly what level of supply protection is needed. It is fair to say it is sometimes not done quite right...

Mike.

Thanks Mike, I appreciate your response. Regarding the issue of the generator failing to start after being idle for a long time, we conduct annual load banking tests, but they incur a high expense as we have to hire a backup generator in the event of a power failure during the 2-3 hour testing period. Furthermore, there is a lot of planning involved in obtaining the permits for the work to be carried out, etc. I am not familiar with the rotary system, either. Does it also require annual load banking?

Surely a simpler way to test the system is to start the generator, synchronise to line, then increase the excitation voltage until the generator is nearly fully loaded. Quick, functionally realistic, value for money, and importantly is not a waste of our precious carbon resources. 

Surely a simpler way to test the system is to start the generator, synchronise to line, then increase the excitation voltage until the generator is nearly fully loaded. Quick, functionally realistic, value for money, and importantly is not a waste of our precious carbon resources.