The emergency public address systems I am referring to are 'voice alarm' systems, which here in the UK at any rate, are increasingly used instead of the more traditional fire bells -especially in larger venues. As such, they are required to meet the appropriate British Standard - BS5839 Pt.8 - which specifies battery support as a secondary power supply. In many cases, these large building do have back up generators as well, so the battery support is often unnecessary, strictly speaking.
But that's what the standard says, and that's what the insurers insist upon!
It's not only amplifier efficiency that is important in helping to keep these support supplies to a reasonable size. The same standard demands that the whole system has to run, on batteries, fully fault monitored, for 24 hours (sometimes for 72 hours!) followed by a further hour at full power, at the end of that period.
So the quiescent current drain of the amplification system also has to be kept to a minimum as well.
But that's another story........