rocky ground eh ? That makes it a LOT harder to get a proper ground. Ask a local electrician what kind of tricks they use. There are machines that vibrate a groundrod in (if it's loose rock, if it's bedrock it's even harder). A possible way would be to situate your mill near a well head and find a ground on that (rebar).
I got lucky here, I have my tower (no windmill yet, but working hard on one) on a hill which is literally full of rocks, and on the first try my groundrod sank in all the way.
Wellheads are usually on lower ground though, and you want your windmill as high as you can get.
One way to look at it is that you gain almost as much as you lose, your ground is going to be hard to make, but putting up the tower & guys is a breeze, you can simply anchor in bedrock.