question

@Kristof G. hello from Colorado. A PV inverter connected on the AC IN side of a multi (or Quattro) is unable to charge the battery during a power company blackout. All it can do is defray the utility bill...and the multi does float charge the battery using some grid-available (mains-available) wattage so long as there is no blackout.

If there is a blackout, the grid connected PV inverter (on AC IN) stops working immediately since most PV inverters cannot create an AC waveform independently.

If the PV inverter is connected to the AC OUT of the multi or Quattro, the Victron blue box is "grid forming" & it fakes the PV inverter to keep running even if there's a blackout, but there are two tricky bits when AC coupling solar to battery in this way:

(1) The compliance of the PV inverter with your nation's requirement for safety on the mains may no longer work quite the same as per its original engineering, so there are safety & compliance issues to consider.

(2) Even though the Victron blue box is an inverter/charger--the easily found charge adjustment menus do not directly manage the PV inverted wattage which is back-feeding AC Out...so the installer must follow protocols & suggested methods to feather the wattage as the battery state of charge goes high.

AC coupling requires a paradigm shift from what we otherwise know about solar plus grid plus batteries. Here's a good place to start:

https://www.victronenergy.com/live/ac_coupling:fronius

Some PV inverters other than Fronius brand are in successful use on Victron platforms, while others are not well suited for a Victron based microgrid project.

Have fun & be safe.