There are two options. Install the fronius on the AC distribution on the INPUT side of the Quattro, so it only works in times there is AC input avaiable. Or have it connected on the AC out Acdistribution which means it works also in inverter mode. If doing so, make sure the PV assistsant ( which can throttle down the fronius when the battery is full) is installed.
Further to @Johannes Boonstra (Victron Energy Staff) answer:
This single line diagram shows conceptually where it can go:
This one is my preferred location when off grid (and DC solar is not part of the system as well). As it means the loads will be disconnected before the inverter shuts down, and allows the system to automatically recover when the sun comes out again.
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