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)
Losses with long distance AC transmission to markets imposes a heavy penalty on remote QC and Manitoba stations. DC transmission is on the way, and has already been implemented in a few places.
That we did not buy all of Manitoba's recently completed Nelson River power (and there's potential for one more ~1200MW station, plus the stalled Conawapa) is more evidence of Ontario being a failed state.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
