fafrd, This might do what you are looking for.
This is a 1000 watt that will accept 22 to 65 volts DC and drives 1120 volt out. So you just put one on each leg of the 120/240 split phase. No Autotransformer needed. Each one will just limit it's leg to zero export. It is meant to work direct from a solar panel, but reading the reviews, there are some people running it from a battery bank. I have not used these, so I have no idea how good they might be. You may need to add some kind of timer control to keep it from running down the batteries overnight.
I finally a manual for these Sun GTIL inverters and I’m starting to get more comfortable with the 120V model.
The maximum output current of the 120V model is explicitly specified at 2X the maximum output current of the corresponding 240V model, so that goes a long way to alleviating my concern that output power is reduced by half at 120 versus 240V (though I still have my doubts about the ‘autodetect’ feature).
Stacking is complicated for non-grid-tied inverters because they need to be synced, but making a split-phase GTIL by stacking two out-of-phase 120V GTILs is straightforward because both units are already being synced to the grid.
Explicit mention is made of battery power in the manual. It seems as though the 24V model will just drive it’s MPPT voltage into a battery as though it were a solar panel. And unfortunately, the manual also makes clear that maximum output is reduced under battery power (to 700W for 1000W model driven by 24V battery).
So two of these 1000W/24VDC/120VAC, each with it’s own limit sensor tied to one phase/leg and tied to that corresponding phase/leg with a common neutral should allow me to offset up to 1.4kW of peak-period consumption without needing the two Autotransformers.
The only thing I’m slightly worried about is that the documentation shows stacking with seperate batteries (following the architecture of stacking with seperate PV arrays, which is necessary with seperate MPPTs).
Thinking through how MPPTs will function with a battery, however, I’m pretty certain it should be possible to run 2 or more of these Sun GTIL inverters off of a common battery, as long as each has it’s own high-capacity cables connecting to the common battery leads.
At this point, I’m down to deciding to purchase through Amazon for $300 each with increased chances of a refund if the product does not work off of a 24V battery as specified, or deciding to purchase through AliExpress for $180 each with shipping and no recourse if I get junk.
I don’t need to worry about a timer since I’ll have plenty of solar current to recharge every morning. In fact, the self-regulating aspect of the system is one of the attractive features to me. Here are the different ‘built-in’ phases:
Off-Peak Empty Dark: running house off grid until sun comes up.
Off-Peak Empty Bright: running house off of grid-coupled AC solar while battery recharged off of off-grid DC solar (house consumption always negative in this phase, so GTILs dormant).
Off-Peak Full Bright: MPPT charge controllers shut down off-grid DC solar production.
Peak Full Bright: as long as AC-coupled solar production exceeds house consumption, GTILs will remain dormant. If/when house consumption exceeds AC-coupled solar production, GTILs will kick-in to compensate net house consumption from battery energy (and MPPT charge controllers may reactivate DC-coupled solar charging depending on available solar energy and battery State of Charge).
Peak Full Dark: after PV production has ended, GTILs will offset up to 1.4kW of house consumption from battery energy.
Off-Peak End-of-Charge Dark: when the battery voltage drops below GTIL minimum voltage requirement of 22V, the GTILs will shut down and house consumption will revert to being supplied by the grid (so back in Off-Peak Empty Dark mode). This will be true whether End of Charge occurs during peak window or during early morning hours after that. In the case consumption was so low that the battery never drains before the sun comes up, nothing really changes except Off Peak Full Bright is reached more quickly.
I like how it’s self-regulating, redundant (parallel to grid), and doesn’t need lots of little additional components like timers or switches to make it work - it will just naturally start using battery energy to compensate for net consumption once net house consumption switches from negative to positive as the sun goes down...