Not just the excess. RECs are used as a way to offset fossil fuel use (think people, companies, places with commitments to green energy). There is a market for each MWh of energy produced. So if you produce 10 MWh of electricity, even if you use some of that yourself, you can sell the REC for cash.
Now if you're solely into solar for the green aspect, you can't then say your energy usage is green because you traded it to someone else and the electricity they traded you would be "dirty."
I'm adding a ~15 KW array to the house this summer. More or less the loan is a wash with our electric bill (which we're going to pay regardless), so it's a net zero for 12 years, and the back half of the panel warrantied lifespan is all gravy. After paying off the solar loan, in the end the net should be around 35k that we don't have to spend on electricity.