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Ontario’s system remained reliable amid significant demand growth in 2025

Total electricity demand in Ontario increased in 2025 by about 4.4 per cent, or 6.2 terawatt-hours, to 145.6 TWh from the year before. Weather was a significant driver of demand growth while tighter supply conditions were the result of planned shutdowns of multiple nuclear units. Demand in Ontario was higher in the summer compared with the same period in 2024 due to prolonged high-temperature conditions, while an earlier start to harsh weather conditions in winter 2025 also contributed to this significant demand. Meanwhile, demand from Quebec increased as a result of low runoff flowing into their hydroelectric reservoirs in recent years, which drove a 10 per cent increase in exports in 2025. In a year of increasingly extreme weather conditions, the strong reliability position of Ontario’s system helped to support the needs of neighbouring jurisdictions. As a result of these conditions, natural gas was increasingly relied upon to ensure reliability across the province.

Due to continued economic growth in Ontario, demand growth in 2025 coincided with the connection of about 385 MW of new load to the transmission system across nine industrial projects. Most of this new load was in the steel and manufacturing sectors, as well as emerging demand from data centres.

An Eventful Year

The year 2025 marked a new era for Ontario’s electricity sector with the launch of the renewed electricity market. After much preparation and planning, on May 1 the IESO launched enhancements to the wholesale electricity market that improves the way electricity is scheduled and priced in the province. Upon the launch of the renewed market, the Hourly Ontario Energy Price was retired and replaced with the Ontario Price. With the launch of the renewed market, the Ontario system began to see more transparent pricing that reflected actual congestion costs and real-time conditions on the grid. The increased price transparency at Ontario’s interties has improved competition for energy trade with neighbouring jurisdictions.

It was also a milestone year for Ontario’s electricity supply mix, which saw the first battery energy storage systems (BESS) connect to the provincial grid including Oneida Energy Storage, Tilbury Battery Storage, York BESS and Goreway BESS. The Oneida facility was the first among these projects to begin commercial operations, and it came online in time to serve system reliability during the first summer heat wave of 2025. As more storage secured through the IESO’s long-term procurements comes online, Ontario’s diverse pool of resources will widen, delivering more value and reliability to the system.

In a hot summer that saw multiple heat waves, the extreme weather conditions highlighted the strengths of Ontario’s diverse supply mix, allowing the IESO to demonstrate its operational excellence throughout the season; in 2025, the system surpassed the province’s highest peak demand for 2024 seven times. Summer system conditions were managed with 10 activations of demand response resources secured through the Capacity Auction. At the time of the system peak, on June 24 – at 24,862 MW, it was the single highest peak in 12 years – these resources resulted in demand reductions of 316 MW; further demand reductions on that day were realized through programs like Peak Perks (200 MW) and the Industrial Conservation Initiative (1,600 MW). For 2025, consumers lowered demand by up to seven per cent over peak periods using these mechanisms.

Capacity

Grid-connected capacity in 2025 totalled 37,760 MW.

Capacity Auction

The IESO secured 1,832.8 MW of supply for summer 2026 and 1,125.3 MW for winter 2026-2027 through the 2025 Capacity Auction. The Capacity Auction complements the IESO’s medium- and long-term procurements by functioning as a balancing tool, awarding one-year obligations to fill short-term gaps. With electricity demand increasing, coupled with pressure on supply due to expected nuclear refurbishments after 2026 – namely, the Pickering “B” units – this annual auction is critical to supporting the reliability of Ontario’s system as grid conditions change from season to season.

Resource Adequacy

In 2025, 555 MW of new capacity was added to the system. More than 80 per cent, or 451 MW, of this additional capacity was from storage, while the remainder came from natural gas.

Enhanced Energy-Efficiency Programs

In the first year of the 2025-2036 Electricity Demand Side Management Framework, the IESO estimates it saw 1,867 GWh in energy savings and 336 MW in peak demand reduction by the end of 2025.*

The Peak Perks demand response program, which expanded to small businesses in 2025, saw 10 activation events of the approximately 280,000 smart thermostats that were registered in the program as of Q4 2025. Through these activation events, Peak Perks reduced electricity demand by an average of 210.8 MW per event.

* These figures are preliminary values that contain estimates and are subject to evaluation, measurement and verification.

Demand, Supply and Price

Here is how Ontario’s electricity system performed in 2025:

Note: Due to rounding, percentages may not add to 100.

* The total output for Storage includes all IESO-metered storage resources. Please note the data reported under “Other” in the Generator Output and Capability Report and the Generator Output by Fuel Type Hourly Report do not currently include output from all storage resources.

† Denotes the Hourly Ontario Energy Price (HOEP).

* The HOEP was in effect until May 1, 2025, and was replaced by the Ontario Price following the launch of the renewed market. The average weighted price for 2025 was calculated based on four months under HOEP and eight months under the Ontario Price.