https://live.gridwatch.ca/home-page.html
As I post this, nuclear is the largest contributor.
The Bruce Nuclear Generating Station is the largest in North America:
https://en.wikipedia.org/wiki/Bruce_Nuclear_Generating_Stati...
I believe they are looking at building a Bruce C to add even more capacity.
There is extensive long term refurbishment of the existing reactors that includes uprated capacity, but there isn't currently any plans to add new reactors at Bruce.
A different Ontario nuclear site (Darlington) does have work underway to build four BWRX-300 reactors, one of which will be the first in the world.
https://www150.statcan.gc.ca/t1/tbl1/en/tv.action?pid=251000...
https://app.electricitymaps.com/zone/CA-ON/12mo/monthly
(75% of Ontario's trailing twelve months electrical consumption was generated by low carbon technologies, the remainder is the gap to fill; adjacent Manitoba and Québec grids stand at 90%+ low carbon for the same time period)
I've had a few friends run into this issue with systems almost frying inverters because they didn't allow enough headroom for open circuit voltage coming from the panels.