The Enphase AC battery could be a big thing but they don't seem to promoting it for grid-tied battery backup or off-grid.
From their web site - https://enphase.com/en-us/products-and-services/storage
What’s your primary energy goal?
Since the Enphase battery has not been released yet, it is difficult to know exactly how it will work. Enphase might not know exactly how it will work.
They are promoting their battery to work with the grid, so it may not work when the grid is down. When you have a battery that will only work when the grid is working, it will make a lot of sense for systems that are not allowed to spin the meter backwards or if there is a big difference between the credit for spinning the meter backwards and buying from the grid.
These types of systems that work with the grid will make no sense in an area, where we have net-metering policies where we get 100% credit/retail value for the energy that we export. Perhaps they can make some sense if there is a time-of-use rate schedule, but it still might not be worth the hassle and the price of replacing the batteries.
In many places in the world, there are not net-metering policies where you can get a good incentive for exporting to the grid. In these places, it makes sense to have this kind of battery system.
Since Enphase is not promoting their battery and are recommending a "legacy" system for people that would like off-grid capabilities, then I would assume that their battery will not work off-grid. I don't blame them, because the off-grid market is not their focus, it is a very small percentage of the industry and it would make their systems much more complicated and expensive.
If you want a more cost effective grid-tie battery backup system, get a dc coupled multi-modal inverter to get the best bang for your buck. If you want to use microinverters, then get an ac coupled system.