It depends on the battery system and how the backup side has been designed.
Tesla Powerwall 3 — With Tesla's backup gateway, every circuit in the house can stay live, provided the system has been designed around the likely load. Without the gateway it won't provide backup, so check it's on the quote.
Sigenergy with the Sigen Gateway — This can be configured either for whole-home backup or for prioritised essential circuits. 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.