kram @AmandaHuddleston There's a separate Emergency Power Supply (EPS) output at the bottom of the Hybrid inverter, next to the normal AC terminals. The Givenergy installation instructions provide a number of different ways in which the EPS output can provide energy to the home (max 2.5kW) from the battery during a grid disconnection. The connection is normally live, fed through from the grid from within the inverter, but limited to 2.5kW. When a power cut happens, the inverter disconnects itself from the house consumer unit, also disconnecting the EPS circuit. A few seconds later, the EPS connection becomes live, fed from the inverter and battery. When the grid comes back on, the EPS disconnects, the inverter connects itself to the grid and the EPS circuit also becomes live again, fed from the grid.
If you are aware of the options before installation, it's just another part of the job to connect your chosen circuits to the EPS either directly or via a manual or automatic changeover switch. However, connecting larger loads such as a kettle or electric heating would reduce the time your battery could support the load. If you just had lighting and maybe a gas or oil boiler circuit, it would last longer.
In our case, we asked the installer to put a 13A switched outlet fed from the EPS terminals next to where our broadband is plugged in so we can plug it in if there's a power cut. We can also run an extension lead from the boiler to the socket if we need the heating on.