Lake Ontario has about 393 cubic miles of volume. To raise the temperature by 1 centigrade you would need about 10^18 joules. So about 455 billion kilowatt hour

Lake Ontario has about 393 cubic miles of volume. To raise the temperature by 1 centigrade you would need about 10^18 joules. So about 455 billion kilowatt hours. I don't believe the cooling needs of Toronto will impact the lake that drastically.