FWIW, I presently use one of these 100amp transfer switches on one of my solar inverters. https://a.co/d/004nY6J7 . I find it reliable and it pulls no current after switching because it latches. So unlike a standard relay it doesn't draw any power when it's "on".

So... more additions to the house. Since my power company stopped paying me 1:1 for the power I export, I'm now going to switch my carport from grid to house as needed. Recently it dawned on me that in order to pull 20kwh from the grid at night, I have to generate 50kwh during the day. Screw that! I'm going to stop using the grid to export my excess power and use the grid only as an emergency generator.

I have two of these transfer switches to be wired in series. One will select between "house batteries" and "grid import" and the other one will simply turn power to the carport on/off. Note that these switches are 6 DIN rail units wide so a 12 slot box will fit two of them perfectly.

The plan is to have HA monitor the car's SoC and when it gets below 40%, I'm going to put the car port on grid power and allow the car to charge to 40%. Once we are there, I'll allow the car to charge from the house system directly only if the house battery is >70%. I'll also hack into the Tesla wall charger Rs485 port and command the EVSEs in the car port to raise and lower the permissible charging amps to the cars. That way if the house is pulling more power than the inverters can supply, I'll slow the cars down. Since I can command each Tesla charger individually, I can priortize filling one car over the other.

A neat thing about my solar system is that it's AC coupled. This means that although my battery inverters can make 12kw, my solar inverters can make 22kw in total. This means during the daytime, I can charge the cars at significantly more power than the 12kw that my primary inverter makes.