Not an electrician, I always imagined the downed power line being close to the house, such that when you backfeed the transformer it doesn't have a load on it until the lineman grabs it and becomes the load.

I imagine when your circuit that's still connected includes several houses or most of your neighborhood, then maybe you'd overload the generator? 🤷‍♂️ I don't know.

Solar systems I do know, though. If you have a grid-tied solar system, that feeds power back into the grid, then when the power goes out, your batteries and inverters are all turned off automatically so they won't kill linemen. That's what's ironic about grid connected solar, you can't use it during a power outage. They only work when the power is on.

Now if you have a standalone solar system, with a transfer panel where you can switch several circuits in your house to either run on the grid OR solar, then you can use solar whether the grid is up or down. But you can never feed the grid to make money back directly in that fashion.