Technically it should do the switching automatically but I have seen it where it has not.
No, the A320 unfortunately doesn't have any kind of Auto ESS Bus-Xfer
After the Incident with
G-EUOB my company added a note in the book, saying that in the event of a major ELEC faiure where the AC ESS FEED Fault Light ON, the crew MAY decide to select alternate ESS FEED outside procedural guidance in order to quickly improve the overall electrical status and to regain COMs.
Regards, MAX reverse