Hedges: I think we are going to buy Sunny Island. Just one, and use with an autotransformer. I want our basic system to heavily focus on battery backup, and try to get to the point where we can run at night on batteries, and recharge the next day. I did think about adding a grid tied portion too, and use Sunny Boy for that. Trying to avoid roof mounted panels.It seemed I got more information on how Sunny Island worked in the 2000's, more theory, diagrams, and explanation.
It works very well for what it does, pass-through from grid and interacting with with AC coupled Sunny Boys off-grid. Some things like load-shed relay are needed and addressed deep in the manual, but conspicuously absent from the system block diagram, which would be the easiest way for people to see and understand.
Sunny Island has been used for load shaving and grid support, but maybe only with custom communications, and in an R&D environment. I haven't seen anything indicating SMA supports that application.
Now, SMA seems to focus on decentralized control and having customers sell power into the spot market. More internet-connected boxes and communications with GT PV inverters. That may be where the grid and net-metering is headed, but it isn't what most of us do.
If you are doing an off-grid or grid-backup system, the basic block diagram is shown in their data sheet, and there are relays available for some functions.
Of course, lithium battery support is officially a number of commercial products. And we know REC is available for DIY.
From what GXMnow has stated about the XWPro being able to do in off grid mode, it does seem like that would be a good option for us too. Maybe even the Conext SW.