AC specifically has such a variable amount of load and requirements to start, they cant plan for "future A/C" unless there is a specific make / model that you are in process of installing. 2 powerwalls might work for some A/C, and not for many others. I understand the desire to have A/C on the backup side, but if your home does not have A/C now, you obviously live in a location where A/C is not "a requirement".
You also likely dont know how fast AC would drain your powerwalls. You would likely not use them for very long if you were in a power outage. With that being said, yes I understand wanting A/C backed up so you have the choice. You will need to install it before your tesla install if you want it on the backup side, they will likely not install it later. Other electricians are likely not going to install it on the backup side later either.
See a picture of my backup loads panel below, which includes every load in my home except my EV charger, and that was excluded by my choice because the load balancing feature didnt exist then and I didnt want any chance of car charging draining powerwalls. Red sticker added by tesla for permitting, it as not an option:
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I actually have (2) A/C condensers connected to my "backup loads" panel, which as I said above, includes every single load in my house except for the tesla wall connector, by my choice. I live somewhere hot, where A/C is "required" (in every home, and apartment), so wanted the A/C backed up. If a home is over 3000 square feet in this area, it has to have 2 A/C units. Mine came with the home and one is for upstairs and one for downstairs. This home is not nearly the size you mentioned yours is, but is 3200 square feet, give or take, so its not "small" either.
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Since these are the AC units that came with the home, built in 2005, they are "builder grade" ones that are not very efficient at all. They are 3 ton units but as the picture below will show, they are both on the backup loads panel. How long I could run them is another thing. Because each of these takes close to 4kWh of power, thats roughly 8kWh of power used every hour. That means if I had them both on, with the rest of my home, it would be 2 1/4 to 2 1/2 hours from full powerwalls to empty.
We almost never run both units at the same time, even during normal operation. In a power outage, they would only be used in 10-15 minute increments to keep the home from being "unbearable" inside.
In a power outage, if it happened in the middle of the summer, having them backed up means I could use the A/C to "increase my load" so the PV doesnt shut off, and I dont even need to try to put that energy into my vehicle (which I current cant because my charger is on the non backed up loads side).
So, I get wanting to have them on there, even if you wouldnt use them much, but
@Notadog is going to have to get A/C installed first, or not have it on the backup loads side. I dont think there is an option for "pre plan for my A/C, when there isnt at least a purchase of the AC unit in progress.
If
@Notadog checks these boards, they will find many instances of people being promised an A/C unit would work with a backup (specifically with 2 powerwalls) then people finding out that soft start units dont work with their specific A/C, or that it doesnt work properly. There would be no guarantee that it would work "later". If its a "must have" then it likely needs to be done before the powerwall install.