This is the only one who comes close to seeing my question.
Oh I do understand that if you can export to grid it requires being in parallel with grid, certainly. And I do know all the definitions above, and well understand microinverters. What I don't understand is how there can not be some sort of interlocutor which can monitor the system's AC production and the grid's supply, not just to keep them in phase, but in a blackout to crowbar export to redirect locally, and with restore to rephase the system to the grid.
And in so doing I do not understand how priority can/can't be given to local use first, whether to load or batteries, with residual then going to the grid.
My (interim) goal is demoting the grid for marginal use only. Long-term goal is divorce.
This is a long post, sorry, but I think the OP might find it helpful.
My system started as a grid tied Enphase microinverter based system. 6 years ago, hybrid battery systems were way too expensive and my utility still had 1:1 net metering, but they sort of hid the whole time of use thing until my system was installed.
In the first year, I saw what was going on and wished I had batteries. Even though I was producing about 70% of my energy demand, I was only saving 30% because of the time of use billing. The evening energy after the sun went down was costing me near double what I was getting credited for extra power while the sun was shining.
I went AC coupled and added a Schneider XW-Pro inverter. Nope, it was not perfect, and it still has some issues, but it did cut my electric bills to where I was then saving the full 70% I expected. The software is lacking, even with a few updates, it still does not know how to charge properly from extra AC coupled solar. For a full year, I was manually starting a charge cycle when I left for work. It would charge 10 KWHs into batteries, and then discharge that 10 KWHs to the grid during the high time of use block in the evening. The XW-Pro also has the transfer relay built in so it disconnects when it detects a bad grid and powers a backup loads sub panel. My Enphase microinverter system is in the backup loads panel and stays working now when the grid is down.
Electricity is a lot like water. It flows through the easiest path. When the grid is up, any power produced by my Enphase solar panels, or inverted by the XW-Pro inverter from the batteries, is just pushed to the breaker panels in my home. That power will run the loads in the house. But it is a balancing act. If the house needs more power than the inverters are supplying, then the extra power will come in from the grid and the meter counts up consumption. If the Enphase inverters and/or the Schneider inverter are making more power than the house is consuming, the extra power flows out to the grid, and the utility power meter counts this on a separate display as exported power. With my current 1:1 net metering, they just bill me based on the difference between the two. But they have the separate data so they could credit me at half rate or whatever in the future. The new NEM 3.0 plans do this, but I got in under NEM 2.0
The real trick is balancing the battery charging and discharging. I have not been able to see how this is programmed in the newer All In One systems. In my Schneider XW-Pro, it SUCKS! Several of us XW-Pro owners have had to make our own system to control this. When all of my solar was just the Enphase system, this was a very big deal. I have a PLC (Programmable logic Controller) that I coded to take the power readings from the XW-Pro and calculate how much battery charge current would zero the grid power. It updates every 5 seconds. More sun, or less load in the house, and it raises the charge current to virtually zero the power exporting to the grid. Reduced sunlight or an increased load in th house, and it lowers the battery charge current to keep the grid power close to zero still. I have it biased to always leave just a little exporting most of the time. But my electric company didn't like it. So now I make it pull a little grid power every morning before the sun comes up, just so they know my meter is working and I use some power. When the sun goes down, the PLC works in reverse. The XW-Pro can do this if you get the approved "Watt Node" power meter on the grid side. It will adjust the amount of power coming from the batteries to zero out the grid current as long as the battery has enough energy. In my system, the PLC sets the inverter current every 5 seconds. My grid power sits at basically zero all day for weeks at a time, just pulling about 100 watts for an hour each morning, and exporting 20-40 watts the rest of the day.
As I said earlier, my Enphase grid tied microinverter system only made 70% of my power. I have since added 2 arrays of panels that are now DC coupled with Victron charge controllers. They directly charge the battery bank. I also double my battery form the original 360 amp hours to 720 amps hours on a "48 volt" (actually 51.8 volt nominal) system. On days with decent sun, the DC panels alone have the battery fully charged before noon, so I usually have the charging from the AC coupled system turned off now. The extra energy from the Enphase system is back to exporting from 9 am to 4 pm. I get more credit from the utility company, and the DC panels make more than I need to run the house all night.
In a perfect world, the new "All in One" units should be able to do what my system does. And instead of the 7 boxes mounted on my wall, it is just 1 box. Efficiency wise, they do end up pretty close. When I was AC coupled only the round trip efficiency was not great. When I was cycling 10 KWHs a day to the peak rate time, I was using 11 KWHs from the Enphase system, so I was throwing away 1 KWH or 10% loss to "time shift" the power to the night. The loss with the DC panels is now cut about in half to just over 5% from the solar panels to my AC loads a night. The microinverters run my loads while the sun shines, and the DC panels provide all the power that I use at night via the batteries. In the AiO units, they use a high voltage DC bus. The MPPT solar inputs drive that bus. Then the batteries use a bidirectional DC to DC converter that can either charge the battery when the sun is shining, or use battery power to drive the bus when the sun is down. The inverter block is also bidirectional and can either take the DC bus and invert it to drive AC loads, or it can pull AC current to drive the DC bus to chare the batteries. This is all done at high frequency with SPWM which is very efficient. The Schneider inverter I have also uses a bidirectional SPWM block, but it is at the 48 volt battery voltage and it uses a HUGE transformer to convert that to/from the 240 volt AC main input/output.
If I had this to do all over again, I would need to research the firmware in the AiO hybrid inverter. How well can it track and adjust the power to zero the grid in both charging and inverting modes? While the XW-Pro does not do it well on it's own, it was not too hard to control it over Modbus TCP with the PLC. The others on this list are doing it with a Raspberry Pi running Node Red and Home Assistant.
