As I understand it from FCOM Vol 1 Electrical page 13, the EMER GEN can be powered by either the RAT or BATTERIES
No, the EMER GEN is hydraulically driven, (only) via the RAT and the Blue Hydraulic.
Take a look at p.3 There it says:
The blue hydraulic circuit drives an emergancy generator that automatically supplies emergency AC power to the aircraft electrical system if all three main genartors fail...
Airbus philosophy is that flight on batteries is only a transient condition, so no way to power the elec system to EMER Level by Batteries.
What are you doing? Transition to A320? If so from which aircraft? Other Manufacturers see 'Battery Power Only' as EMER Level and 'Stanby Generator' as ESS Level, so be cautios to understand AI terminology.
Cheers, MAX