We priced this out when we bought our Ford F150 generator on wheels. It was maybe 3k including parts and labor. We opted to go without the fancy socket and breaker, and it cost us about 600 USD.

The cheaper version even, which may be what you are talking about, is literally just a piece of plastic or metal called an interlock on the breaker box that prevents the main from being on while the generator inlet is supplying power (backfeeding the line). Then all you need to do is install the inlet on top of that which when I did this a few years ago was around $1,000 USD all in parts and labor.

Electricians like their transfer switches because they are right sized and everything, but the interlock approach actually gives you more control because every breaker is allowed (a transfer switch will only expose some small subset of all your breakers usually), you just have to manage the power supply yourself.

Sorry, that’s for a full Tesla Powerwall (which includes its own 13.5 kWh battery) which is required to use the recently-announced Model 3 and Y home backup feature. The Cybertruck only needs the gateway and wall connector, which is still $2k plus labor.

Sadly in CA…