Hmm.. finding a 24v inverter that will allow you to set it for a 18v low voltage disconnect/shutdown might actually be tricky. I would be surprised if you could find one that would allow a setting lower than 20v. I mean, i hope you can, but i can foresee a likely possibility that you will either have to A: leave everything below 20-21v unusable, or B. devise a secondary inverter system to make use of what remains as a sort of 'critical loads' system, or address the 'solar startup' issue as well by C: Substantially or entirely replacing the growatt system with different components.
Are you using ~230vac for all of your loads? Or using an autotransformer to make ~120vac? How many watts would you need to continue powering your 'critical loads'? That would change my idea for what that secondary power system from option B would be.
I see you are using the inverter in a skoolie. I imagine that if you are not just trying to upsize the system to use AC all night, then your critical loads that you would like to stay running even when the Growatt is currently shutting down, are probably no more than few hundred watts and might be under 100w. IF that is the case, you could do something as simple as buy a solar generator which will take up to ~28-29v on its solar input, hook your tesla batteries to that solar input, hook your Growatt AC output to its AC charging input, put it in UPS mode, and power your critical loads off of its AC output. That would essentially have your tesla batteries keeping the solar generator fully charged at all times (because its solar input would probably accept anything between ~12-29vdc and step it up to charge the internal battery), have its inverter be basically idle because it sees the ac input from the Growatt and passes it through to the output for your critical loads, and then when the Growatt goes down the solar genny UPS thingy will start powering loads from its internal inverter and battery. If your critical loads 'draw' was lower than your charge rate from the Tesla batteries, you would get all the Tesla batts remaining capacity + the solar genny battery capacity (small) before your critical loads finally shut down. If the draw was higher than the "solar" charging from your tesla batts, you would get something in between before it all shut down. This is basically using a solar genny as a UPS that happens to accept DC charging from your tesla pack. It is perhaps not the most elegant solution but if you have only a small amount of critical loads it may end up being pretty cheap and you still get to carry the solar genny around as a separate little useful toy if you ever cared to. You could accomplish the same thing with an actual UPS that used a 12v battery, and hooked a 24 to 12v (preferably 13.8) step down converter to charge its internal batteries from your tesla pack, but then you wouldn't have a solar generator as a toy. Lol
