At our field (And it's supposedly EASA standard in that regard), all navaids, VDF/UDF, radar, radios, all equip. in VCR / APP, and essential radio links. (Radio links to mountain top transmitter sites etc) are on UPS. (Several separate)
Airfield lights are not on UPS, and max startup time for the generators are 16 seconds. Due to that, when visibility drops below 800 meters we start the generators that power the lights, and run the outside main as backup. Switchover time from generators (should they fail) to mains power is 1 sec.
One generator power every second light on the runway and in the approach light systems. Other lights (TWY, signs etc) are split in half geographically.
That way, you still have limited function with only one generator and no mains.