[thrtythreeforty]

Suppose I own my house and don't feel like running a new set of wires thru my house, so I'm comfortable separating the panel into a downstream solar-capable subpanel and a real battery stack, but want a similarly thrifty and red-tape-free setup (which never backfeeds the grid). What other pieces of equipment could I substitute?

[toast0]

You can pretty easily setup a subpanel with a feed-in outlet and an interlock so you only have the feed-in or the grid breaker closed, but not both. Something like this [1], but you need to shop to match your panel.

That's thrifty; changing a panel/building a subpanel probably needs a permit, but shouldn't need utility approval. A transfer switch is an option too, but not so thrifty, and if you an automatic transfer switch, they usually work the other way (use grid if available, fail to local generation), but you could probably make it work.

[1] https://www.geninterlock.com/product/generator-interlock-kit...

[reilly3000]

Get a transfer switch. It’s a 30 minute install by a utility or electrician.

Victron multiplus ii cx could be a good option.

[Rebelgecko]

Depending on where you live, systems that you just plug into the wall are legal up to a certain wattage as long as they don't backfeed into the grid