Sunshine hit send before I could...
I am running a system with AC coupling, so I can give some first hand data.
There are a few issues with AC coupling. In my case, I am running the older Enphase iQ7 microinverters. Many people say they are actually a little more forgiving to get working with AC coupling. But this changes with every software update. You also need to have a grid code loaded that will work with AC coupling.
As you have already mentioned, grid tied inverters like the Enphase units need to see a "good grid" to be able to produce power. Though, since you do have iQ8's, they actually can grid form on their own. But it does require the Enphase Empower gateway switch. When the grid goes down, it will disconnect from the grid and tell the iQ8 inverters that it is safe for them to start making power. This obviously can only provide power while the sun is shining. And in the case of AC coupling, they also can only make power with sunshine. The big difference is that a battery can keep making power when the sun goes down, and even when clouds pass over. Upgrading your system to daylight backup may help and you can still use an Anker or whatever battery system to keep you running after sundown. But if you spend a little more, you can also add the 5 KWH Enphase 5P battery.
But you are asking here, because you don't want to spend Enphase prices on this stuff... Right?
For a battery inverter to be able to safely grid form and make grid tied solar inverters power up, there are several important requirements. While almost any inverter might make a good enough sine wave for the inverters to try and come on, things can go very bad. I am just going to walk through the steps of turning on an AC coupled setup. Step one is you need to be disconnected from the grid. Using a generator transfer switch should work here. Step 2, the battery inverter has to power up and start running the loads. Let's say you have a few small things turned on. The inverter has to power up the refrigerator and a few lights. Ok, you are running on battery. Step 3, after 5 minutes of a good grid, the grid tied solar inverters will try to start powering up the system. In a perfect world, the PV solar power comes on and is supplying the power needed.
Step 4 is where we have our first issue. The PV solar panels start making more power than the loads need to run. The extra power will start to flow backwards into the output of the battery inverter. The Grid Tied inverters don't know they are not on the grid. They are just trying to push all the power they can from the solar panels. Many inverters at this point will shut down, or even be damaged. The inverter needs to be designed to handle bidirectional power. While some inverters can do it, they may not be very efficient at the conversion, and they will get hot and can still be hurt. DO NOT DO IT unless the inverter specifically says it can do AC coupling. Even if it looks like it is working at first, it could blow up at any time as it is doing something it was not designed to do. If it can do it, it will be advertised as a feature.
Step 5 is yet another problem. It happens to work, and the battery is being charged by the back fed power. What happens when the battery becomes full? An inverter that can do AC coupling uses a feature of grid tied inverters, "Frequency shifting". As the battery reaches full, the inverter/charger will raise the frequency. On some modern grid tie inverters, this will cause the power to be reduced slowly and it can find a balance point where the battery stays full. But in many cases, the frequency shift might go far enough to where the grid tie solar inverters just turn off as it is now a bad grid. The system runs off battery, the battery runs down a little, the frequency shifts back to normal, and the grid is good again, and the cycle repeats. Without this function, batteries will over charge.
And the bonus round, the battery system and inverter needs to be able to handle the entire power of the solar array. If you have 5,000 watts of solar panels, your battery needs to be able to accept 5000 watts of charge power. This is because the back fed power needs to be controlled by the battery inverter under all load conditions. On a 48 volt system, this is still over 100 amps of charge current. You can't get away with a small battery.
