From what I understand there is a lot of variation in providers. As I understand SDGE's Net Metering policy(https://www.sdge.com/residential/solar/getting-started-with-...), you sell back power at retail prices only for the current month's billing cycle. Excess energy created within a billing cycle is "true-upped" at wholesale prices which can be applied to other month's billing cycles.

I would also be curious in how the Texas case works. Especially if the grid is down, would it be able to accept the energy you are producing?

The rules in Illinois are you net meter for the year. If you make more than you use in a year that is free power for the utility. If you want a different deal you need a contract with the utility (In general you need to be in the millions of dollars/year range to be of interest). In Texas it is more complex as each provider can give you a different deal for residential systems.

When the grid is down you can't sell power back. The whole system shuts down. Though if you have a whole house battery backup you can use that instead of the grid (if it is built for it - solar gets weird if you aren't using exactly as much power as you make so you need something to use or make up the difference).