Yep, I agree that AC coupling is mostly used for retro-fit applications where someone already has an inverter in place, and they want to bring in more power such as adding a new inverter like a Sol-Ark for example (or any inverter that can handle and do AC coupling). The idea is , that you keep your old inverter in place and working (no changes), then you bring that existing AC power its producing over to a new inverter and AC couple it so the new inverter uses that AC power from the old inverter, and more-or-less passes it through onto your loads and/or batteries. That way you don't have to remove your old inverter , and you get to utilize that power. More than likely the new inverter also brings in new power by way of new additional solar panels (a 2nd array for example).
But there's some people (like myself) that might have say extra 4,000 watts of solar panels in storage that they want to use. I could buy another MPPT SCC and bring that power over to help charge my batteries but my existing array and inverter are doing a nice job at topping my batteries off within 3 hours each morning. From my knowledge not all inverters can AC couple with another inverter (whether existing or new). I have a Sol-Ark 12K and I could utilize the 4kw of power from those panels by AC coupling it to my Sol-Ark. That's AC power so the Sol-Ark doesn't have to run it through its inverters because its already AC power coming in.
I talked with Sol-Ark since the user manual just touches on AC coupling. One phone call was worth it for sure. So basically I can bring in 9,000 watts max via AC coupling. It requires that my GEN breaker is used for the AC coupling, which is fine with me because I don't have my generator connected directly to my Sol-Ark. My generator is a stand-by and its connected to my main service panel via ATS. That way if the grid fails, the ATS fires the gen and covers my main service panel, and by way of the 60A breaker I feed the Sol-Ark it's grid power for pass-through. I realize that's not the generator configuration some would prefer but it's what I have because my stand-by generator was already in place when I decided to build a solar system, and the 22kw gen can produce ~92-96A so I didn't want to clip its capacity by connecting it to my Sol-Ark since it can handle 50A into its GEN breaker input.
The reason I want to bring the 4kw of panels power to my Sol-Ark is it can manage the power better than just getting another MPPT. It can use the power to cover my critical loads panel as well as assist my main service panel since I am using the "limited to home" mode which to me is maybe the best feature of the Sol-Ark. All that wiring is already in place so so AC couple another inverter is really easy.
Sol-Ark, as I guessed, would not recommend a brand or model of inverter but said as long as it will do 240V , and is UL1741SA , then it would work. They said if I were off-grid then I would want to use frequency shifting to wake the new inverter up. But , I am grid-connected so they said no worry about frequency shifting, that the inverter would just be either off or on, and it would not have its power adjusted (like it is when you have DC) when coming in, that it would just come in as pass-through AC power. Of course I would be limited to the 63A pass-through when on-grid which is fine because I'm currently no where near that with my existing config. I still wonder if the power coming in from the AC coupling will continue to power or cover my critical loads panel. I think it should but will confirm. I do realize that the Sol-Ark will stop sending any and all power to my main service panel in a grid down situation because by law they can't send power to the main service panel when grid is down due to safety reasons. But I'm 99% sure the power from the AC coupled inverter will continue to be fed to my critical loads panel only.
Verdict is ....my desire to bring in additional 4kw of solar panel power is well within my inverters capacity and current configuration. The Sunny Boy or Fronius seem to be a good fit, probably get a ~5kw model, and install it close to my Sol-Ark so the AC wiring doesn't have to be so long. I can bring PV wire the 180 feet from the array all the way to the new inverter easier than I can running AC wire I would think.