I just found this thread today and saw my name mentioned a few times. I will add my 2 cents here.
From what I understand, backup power is not high on the priority list. In that case, it could make things far easier. AC coupling is really only an issue when you go off grid in a grid failure situation.
If you don't want to use a separate load panel for the battery power, then you need an inverter that can push from batteries to the grid. I know the Schneider XW-Pro does this very well, but as we have reported, it won't start a charge cycle on it's own. The SW line can start a charge cycle, but it can't push back to the grid. It will work only to supply power to loads in a separate panel after the inverter.
The Outback Radian, Skybox, and Mojave appear to have decent software for this and can push to grid, but I do not have any hands on experience about how well they would function.
The large array of Enphase PV Solar is not a big problem. If I had 16 KW of Enphase, I would split the system. If you are not worried about running off grid, then just leave all of the Enphase in the main panel. Problem solved. It won't work when the grid is down, you said that was not a worry. But what if you do have a grid failure? Well, I am sure that much power is going to several breakers in the iQ combiner box. Just take one or two of the strings and have them on a transfer relay. While the grid is up, they connect to the iQ combiner breaker. If the grid goes down, have them connect to a breaker in the backup loads panel. If you have 38 panel to make 16,000 watts, that means 420 watt panels. 6,800 / 420 = 16 x 420 watt panels could AC couple on a Schneider XW-Pro. And 19 x 420 watt panels could go to an 8,000 watt Radian. Choose the two breakers that go to less panels than these limits.
When I first got my XW-Pro, I was eager to put it to use. At first I only connected the AC1 grid input to my main panel, on just a 20 amp circuit. I had 18 KWH's of batteries on it. I se the charge block and the grid sell times to charge while the sun was up and to grid sell from 4 pm to 9 pm during the peak rate time of use. And messing with the settings, I never could get it to charge by itself, but if I hit bulk charge in the morning, it would charge up and go to standby. And when 4 pm rolled around, it would just export back to the panel whatever current I set until 9 pm. The only manual intervention was hitting bulk charge each morning. Talk about easy wiring, just 4 wires L1 L2 Neu. Ground. But this setup would only charge at a fixed rate and export at a fixed rate. It had no idea of power available or load.
Once I installed the backup loads panel and moved over my essential loads, I started to see how good the XW-Pro really is. It still would not charge on it's own, but it now knew the load on the output so it was now able to adjust it's current draw to follow the demand. Since most of my load moved to the backup load panel, I reduced the grid sell to just 3 amps. At 4 pm, I did not see it selling any current, the battery power was at zero. The Enphase Solar, which is now in the backup loads panel as well, was making enough power that it was running all my backup loads and pushing more than 3 amps back to the grid, so the XW sat idle. As the solar fell off, the XW ramped up to keep 3 amps going back to the main panel. That is a great feature. But still, it won't self start a charge cycle. That is a stupid software mistake by Schneider.
I don't know how the Outback inverters work in those situations.
To do a good job of powering your home while on grid, you do need a way to monitor the power being used. Some systems use CT's or current transformers on the grid input lines and with the voltage also being monitored they can measure the power that is going in or out of the system. A smart system can use that data to adjust battery charge rate and grid power export to keep the grid at zero power. It all comes down to the software to make that work. Some systems do it pretty well. Others need some help. 400bird and I programmed our own boxes to force the XW-Pro to do it very well.
