Parece una buena instalación 
@Svarky, Francisco has done a solar course a while back, and installed these panels and inverter. However he is wanting to add batteries.
I believe he wants to know how he should size the batteries and Victron with the 6kW solar system, and also whether to use a transfer sandwich (which sounds like a device to switch between grid and batteries).
Sounds like there’s probably a lot of questions to get to those answers… I was thinking he probably needs to fill out a load sheet, and get in a battery system designer?