This is a simple bill swap. Nothing more.
If your roof is producing all the power your home uses, what's left for FPL to charge you? Nothing — because they're not supplying your power anymore, we are.
That expensive FPL bill, the one that keeps climbing every year at a rate you can't predict or control? We bring it down to $0. Then it's replaced with one bill — a lower, fixed one that doesn't move on you.
No second bill stacked on top. No surprise charges. Just one bill, swapped for a cheaper one.