Electrons don't "stop" at provincial borders but there are certainly limits to the extent to which provinces buy and sell electricity between themselves. Ontario Hydro has to pay Hydro-Quebec for electricity imported from Quebec and vice-versa, and that's an incentive to try not to do it any more than necessary.
On the western side of this province (Ontario), the boundary between Ontario and Manitoba is huge in terms of distance over land but there is one road that crosses the boundary and keeps going on the other side, and I can't imagine that the electrical transmission situation is conceptually much different. It's 700 km from Thunder Bay to Winnipeg with only relatively small settlements in between. (Population Kenora ON ~ 15,000)