Supply Mix and Generation

Ontario has a clean electricity grid with a range of diverse resources, including hydro, nuclear, natural gas and renewables. Each resource generates electricity differently and has unique operating characteristics. Because no single resource can meet all of the system’s needs at all times, maintaining a diverse supply mix is an effective way to ensure the ongoing reliability of Ontario’s electricity system. 

Capacity

Capacity is a measure of the maximum amount of electricity the province’s system can supply at any given time. Ontario’s capacity is constantly changing as new supply comes online, older generators are taken out of service and new technologies are introduced.

Transmission-Connected Capacity

This is the capacity of resources that are connected directly to the high-voltage provincial grid, which is controlled by the IESO. Typically, these are industrial-scale power plants and wind and solar farms that can produce large amounts of electricity. Transmission-connected resources are the backbone of Ontario’s electricity system and they supply most of the province’s energy needs.

 
Transmission-Connected Capacity as of December 19, 2024: Nuclear = 12,184 MW or 33%, Natural Gas/Oil = 10,450 MW or 28%, Hydro = 8,862 MW or 24%, Wind = 4,943 MW or 13%, Solar = 478 MW or 1% and Biofuel = 287 MW or less than 1%.

Transmission-Connected Capacity as of June 19, 2025 (Source: Reliability Outlook)

Distribution-Connected Capacity

This is the capacity of resources that are connected to a low-voltage community grid, which is controlled by your local hydro company. Typically, these are small-scale generators, demand response resources or energy storage that are owned and maintained by individuals, local facilities or other businesses. These resources serve some, or all, of the energy needs of their owners, reducing demand on the provincial grid. 

Ontario’s distribution-connected electricity supply on contract as of July 2022 broken down as: Solar = 2,171 MW or 61 per cent, Wind = 591 MW or 17 per cent, Hydro = 333 MW or 9 per cent, Gas = 320 MW or 9 per cent, Bio-fuel = 110 MW or 3 per cent and Waste = 24 MW or less than 1 per cent..

Distribution-Connected Capacity as of July 2022 (Source: Progress Report on Contracted Electricity Supply)

Energy Output

While capacity represents the maximum amount of electricity that the system can supply at any given time, the actual amount of energy produced varies. For example, while natural gas represented about 28 per cent of Ontario’s total transmission-connected capacity in 2024, it only accounted for about 16 per cent of actual generation. Most of the electricity produced in Ontario is generated at nuclear and hydro plants, which produce low levels of greenhouse gas emissions.

Pie chart - Energy Output by Fuel Type - Year 2024

Total Electricity Output by Source in 2024 (Source: Year End Data)

Different Types of Electricity Generation

  • Baseload Generation
    Nuclear and run-of-the-river hydro plants generate a constant, steady supply of electricity - 24 hours a day, 7 days a week. The output of these generators is consistent and reliable, but rarely changes. Because of these operating characteristics, they are typically used first to meet Ontario's energy needs.
  • Intermediate and Peaking Generation
    As demand rises and falls throughout the day, more flexible intermediate forms of electricity generation are used. Generators such as natural gas plants and hydro dams – which can adjust their output up or down quickly – play a crucial role in matching supply and demand throughout the day. These generators can also be called upon to meet peak demand when electricity use is at its highest.
  • Variable but Controllable Generation
    Wind and solar farms generate more or less electricity based on how sunny or windy it is. While the amount of electricity they produce is always changing, their operation is very flexible and their output can be adjusted quickly in response to the electricity system's needs. 
Different Sources of Electricity Generation

Distributed Energy Resources | Ontario’s Electricity Grid

One of the most significant changes to electricity systems around the world has been the emergence of new technologies that can support locally-owned facilities for electricity generation and storage.