After feed of some electric power into the grid,

You cannot distinguish that current from the rest in the grid!.

But:

There is no problem basically with "longe range".

The transformers connecting the local to the medium voltage

grid will transform the current generated from the

roofs back. Controlling a generator feeding into a

AC grid is not an easy task. Whether the small

generators  control circuits are sophisticated enough to feed

back up to a middle voltage net? I do not know.

Feeding back upward would imply a lot of such generators.

There might be a problem to get them work synchronously

when their combined feed would exeed the power consumed

in the local net.

Georg