One can put the whole distribution board of a house on backup power. But this is not recommended as the amount of backup power and energy is limited.
Typically one would like to exclude high energy consumption appliances that are not essential from being supplied with backup power. Appliances such as stoves, ovens, geysers, pool pumps and underfloor heating.
So one would divide the distribution board up in two sections, or even add an extra DB board next to the existing DB board if there is not enough space.
The one section of the DB or the new external DB would be supplied just with grid power, and the other section with grid power and backup power.
An extra earth leakage protection(ELP) switch is installed in this new DB board. So each section/DB has it’s own dedicated ELP.
Then certain existing circuits can be moved over to the new “backup power DB Board”.
Alternatively, extra plugs can be installed in addition to the existing plugs, that are only for backup power, if one circuit supplies both essential and non-essential equipment.
Example of a dedicated external extra DB for backup power.