Ontario recently started a pumped hydro facility in an abandoned mine, and it will hold ~400 MWh of power that can be charged or released over several days. Several other sites are in planning, including one that, if it goes forward, will hold 8000 MWh. That could run the entire city of Toronto for several days.

Such facilities should allow Ontario to run on a mix of pure nuclear, hydro and wind. Currently about 15% of the electricity supply in the province is natural gas, used at peak times when demand exceeds the constant output of the nuclear/hydro base. At other times, their surplus is sold at a loss. More pumped storage will smooth this out.