It depends on the battery system and how the backup side has been designed.
Tesla Powerwall 3 — Whole-home backup is part of the standard proposition. Every circuit in the house can stay live, provided the system has been designed around the likely load.
GivEnergy with EPS — This can often be configured either for whole-home backup or for a dedicated critical-circuits board. In many homes, the better approach is to protect the essentials rather than try to run everything.
What counts as essentials — Usually lighting, fridge, freezer, heating controls, internet, sockets for charging phones, and sometimes a few other chosen circuits.
Why some homes choose critical circuits only — It stretches the runtime much further. Large electric showers, ovens, immersion heaters, and other heavy loads drain a battery quickly.
We assess the property and tell you honestly whether whole-home backup makes sense or whether a protected-circuits approach is the better fit.