I was told when the KIDD Met site in Timmins shut down it was Ontario's single largest consumer of hydro. From that point on we were exporting electricity some days.
I have a pretty interesting little book here with the Ontario Hydro procedure for emergency load shed for INCO's Stobie mine. All hand written, what loads to dump and time tables for how long we had to do it. I should scan some of it I think you might appreciate a gander. It was a huge consumer of power 4 hoists, several crushing plants, compressor plants, pumping stations and a mill. Nothing saved but scraps of paper now.
I track Ontario's electricity gen and post bi-weekly wind output and Darlington Nuclear output on Twitter so I'm pretty up-to speed on the subject
Ontario demand peaks at ~24,000MW in the summer, less in the winter. Current demand is 19,444MW with a market demand of 22,442MW. We have ~13,000MW of Nuclear, there's 10,000MW of it online right now, so it's very rare that demand dips down to that level (though it does happen in the spring and fall briefly sometimes). For some parts of the year nuclear + hydro can satisfy pretty much all of Ontario's demand, but they wouldn't be able to do it right now. Hydro and available nuclear maxed out would be ~18,000MW so we are burning 3,300MW of gas right now and exporting ~1,500MW of wind (all of it).
Wind produces at ~13% of capacity during the summer, so it is pretty much useless. It produces >40% in the spring and fall when our demand is lowest, which drives up exports considerably and the market rate down, which in turn is why exports have become a massive exercise in losing money for the ratepayer. Historically, before wind, exports were a for-profit endeavour as we could export reliable excess hydro, which is our cheapest source.
Now, all that said, Bruce is capable of curtailing output. It offers up to 2,400MW of "flexible" generation which it can shed in blocks via steam bypass. It used to do it when wind had "first to grid" rights and we'd dump steam at Bruce instead of curtailing wind. Now we curtail wind, but that doesn't fix the cost angle because wind is on fixed-rate contracts, which is ludicrous, but that's the deal McGuilty and Wynne married us to unfortunately.