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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...