There are a few threads on here with people who have AC coupled systems. It seems the iQ8 series is not the best when it comes to running off grid with other inverters. Do you have frequent grid failures and need backup power often? Or do you want to go fully off grid? In those cases, I don't have any great recommendations beyond using Enphase batteries. Yes, they are expensive, but they are designed to work together. Your iQ8 inverters help to form the grid while working with the iQ batteries. When using anyone else's inverter, the iQ8 still thinks it is running on grid, and it seems to aggressively test to make sure the grid is a solid real grid.
While the Sol-Ark may work, they do want you to also have DC coupled panels to make the system more stable. They connect grid tied solar through their generator port, and when the batteries become close to full, it disconnects the grid tie inverters. Not the best solution as you could be forced to be running on batteries while the sun is still shining on the panels.
My system is using Enphase iQ7 inverters. They are not quite as picky as the 8s, but out of my 16 inverters, I typically have 5 of them that just won't stay producing while in off grid AC coupled mode. And the other 11 will randomly stop and restart. My battery inverter is a Schneider XW-Pro. It is a rock solid inverter, but the software has some holes in it. When running AC coupled on grid, it will NEVER start a charge cycle on it's own. The inverter shuts off when the battery drops to 0.5 volts ABOVE the "recharge volts" setting. And then it will sit for days without doing anything. For backup power, this was fine. It would charge up and stay charged, Then if the grid went down, it would run on the batteries, and pull them further down to the low voltage cut. When the grid returns, or AC coupled solar comes in, as long as it pulled the batteries below the recharge volts, it will then go into bulk charge.
A few of us on this forum have added our own controller to command the charge cycle, and that works great, but it is nothing off the shelf. You need to know a bit of programming to make it work. And even after we made it work, we still decided it was best to still add a few DC coupled panels to help with charging.
If you don't have many grid failures, then the AC coupling problems of the iQ8's are not really a big deal. You can do power time shifting while the inverters still see the real grid. The problem is finding a battery inverter that has the software to do it well. I have not seen one on the market that really does it right beyond the Enphase iQ system or a Tesla Powerwall. The new EG4 18K PV inverter says it can do time of use control, but I have not seen how it does it. Before I programmed my controller, I was charging at a fixed power when the sun was up, and discharging at a fixed power during the peak rate time. It worked, but was not ideal. To make it track solar production and energy usage, I had to add current meters. The Sol-Ark and EG4 include them, but does the software use them to control AC coupled charging rate?
Tell us more about what you are trying to do. Backup power is easy. Time shifting for "time of use" and "self consumption" takes a bit more work.
