Early ETOPS in UK with 757 required apu on for ETOPS . Once hyd generator and other ETOPS specific mods were incorporated then it just had to be serviceable for dispatch. Not sure about current status as that was late eighties ! Ref 777 at the moment apu reqd to be serviceable for 207 mins ETOPS, if INOP then you're restricted to 180 mins. Not meaning to be picky but to all intents and purposes you have 2 generators on 777, the back up generators arent treated as normal generators for dispatch, just to clarify for non 777 people as it could be misleading !!