Almost nobody does that, because solar + batteries would need so much batteries for winter that it is completely unpratical. The only people that I know doing that are extreme ecologist in remote places. And grid access is so cheap like 10€/month.
If the grid connection fee is only €10/mo, then that doesn’t sound like it is really covering the full cost. Most US utilities have a monthly fixed fee around that amount. Just checked and mine is $15.05/mo. That fee is basically arbitrary and if everyone went solar and was only buying a nominal amount of energy from the grid there’s no way a few bucks a month from each customer would support the whole thing.
