Systems with EPS (Emergency Power Supply) switch to battery power automatically, typically within around 20 milliseconds. That is fast enough that most appliances will not even notice the grid has gone down.
What actually happens — The system detects the grid failure, safely isolates your home from the network, and transfers the protected load onto battery power. This all happens automatically.
What you notice — In most homes, very little. Lights usually stay on, routers stay live, and your fridge or freezer keeps running without a manual reset.
Why this matters — The whole point of a battery backup system is that you do not need to go outside in bad weather, start a generator, or start switching circuits by hand. It just takes over.
The speed and smoothness of switchover depends on the system specification, which is why we plan backup functionality properly at survey stage rather than treating it as an afterthought.