These portable setups you guys are using has really got my mind churning.
Since I have net metering and my grid rarely goes down I would be happy with a manual switch over to off grid.
I am very confused about why you guys are saying Sunny Island works better with AC coupling.
Sunny Island can be used with DC coupling or AC coupling.
With AC coupling, Sunny Island decides and knows how much current goes into the battery or comes out of it, because it is the only connection.
With DC coupling, it only has measurement but not control.
SMA previously sold "Sunny Island Charger", SIC-40 and SIC-50, which were actually manufactured by MTSE:
SIC50-MPT® - Advanced MPPT solar charge controller Sunny Island Charger 50 for SMA Sunny Island Systems.
www.sunnyislandcharger.com
With the addition of "SIC-PB", its data bus is converted to the CAN bus used between multiple Sunny Islands. Voltage set-points of charging phases (but not maximum charging current) is controlled by Sunny Island, and amps/amp hours are reported.
Midnight Solar's Classic charge controller similarly has a data interface, so it can do the same.
With addition of a battery shunt, any other DC source can be used as well. Current is seen by Sunny Island, but voltage settings are separately controlled by the other sources.
Sunny Island can export to grid (net metering) from DC sources, if the battery voltage is driven higher than what Sunny Island wants. I think that means only sources it doesn't control, i.e. those which require addition of a shunt. And if the grid happens to be down, those sources will hold the battery at a higher voltage, so that setting needs to be low enough to do no harm.
Sunny Island's menu displays up to four SIC, showing PV voltage and current.
Because Sunny Island can't control current to battery from those sources (only choose not to add more from its AC sources), you need to limit DC coupled PV (or other types) to maximum battery charge current. For instance, to keep battery charging at 0.2C, if you have 10 kWh of battery then only 2 kW of PV should be DC coupled. If you're consuming 3 kW of AC at that time, battery gets discharged at 1 kW. If you had 5 kW of PV, battery would get 2 kW and AC would get 3 kW, but when your AC loads turn off battery would get 5 kW which is 0.5C
Sunny Island can manage AC coupled PV up to 2x its own rating. A pair of SI-6048 for 12 kW can have 24 kW of AC coupled Sunny Boy (which can be over-paneled beyond 24 kW of PV.) It will simply adjust frequency to request 5 kW of whatever, so your AC loads get 3 kW and exactly 2 kW is drawn down to charge batteries.
SMA recommends 100 Ah at 48V (5 kW) of battery per 1 kW of PV, probably to handle the load-dump situation when your A/C or well pump shuts off, and suddenly Sunny Boy is delivering more kW than you need. The frequency shift control is relatively slow, takes seconds. Sunny Island can deliver 140A (7kW) maximum charge current but if battery can't accept that, AC voltage could rise to where Sunny Boy disconnects.
I get away with having battery 1/4 the recommended size. I don't have individual loads turning on and off which are such a large fraction of the total.
I would suggest using DC coupling up to the "C" rate your batteries can accept, if you use DC coupling at all. AC coupling with inverters that have an off-grid setting (not just "Rule 21"). Batteries sized according to your budget and nighttime draw or periods without enough sunlight. Because my PV array is oversize, even overcast days should usually produce enough power for some loads like refrigerators. Large loads (A/C, tools) can wait for sunny days.