In the case of Spain, at least, they own the grid, so all solar energy that you sell to distributors because you have no use for it yourself, they'll only pay you peanuts for it and they will still make a devious profit.

The two solar panels companies that I got in contact with weren't interested in selling me a quantity small enough that was coherent with my needs, and they'd charge me a premium if I wasn't willing to make a contract with them to sell them specifically the excess energy.