Ontario’s electricity system operator is preparing for a gradual change in grid capacity as demand is expected to increase by almost 65% by 2050, with peak load rising from about 23-24 GW today to 40 GW in high-demand scenarios, according to the Independent Electricity System Operator’s Annual Planning Outlook 2026.
Planning models show the sharpest increases are concentrated in southern Ontario, particularly the Greater Toronto Area, where electrification of buildings, transportation and industry is driving sustained growth in peak demand.
Individual large loads are amplifying this pressure. Data centers and electrified industrial facilities can add several hundred megawatts at a time, creating localized peaks that existing transmission infrastructure cannot absorb.
Ontario expects 16 more data centers to connect to its grid over the next decade, representing 13% of new electricity demand and 4% of total forecast demand, according to analysis published in Energy Regulation Quarterly.
Power flows bound for Toronto reach transfer limits
The system operator’s reliability framework identifies transmission, not generation, as the binding constraint. The main corridor supplying power to the Toronto area, known as the Flow East Towards Toronto interface, has a transfer capacity of about 5,900 MW, according to grid operator planning data.
Planning materials identify this interface as a limitation during peak summer demand periods, when high temperatures reduce line ratings and demand is high, with limits often set by 230 kV circuits between the Trafalgar Transmission Station in Oakville, about 24 miles west of Toronto, and the Richview Transformer Station in Toronto’s west end.
At the same time, power from the Bruce nuclear complex must move through west-to-east interfaces with transfer capacities in excess of 5,600 MW, concentrating generation flows into the same limited corridor in Toronto.
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Peak demand approaching 40 GW means that the approximately 5,900 MW of transfer capacity in the Toronto region must serve an increasing share of the system load, requiring several thousand megawatts of additional transfer capacity in higher demand scenarios to maintain reliability margins.
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Draft Plans Center on 500 kV lines, HVDC link in Toronto
IESO presented draft recommendations during an April 30 engagement session, selecting Portfolio B with an initial cost of capital of $9.8 billion, compared to alternatives priced at $9.2 billion and $9.3 billion, respectively.
The operator cited Portfolio B’s leading performance in reliability, operability, resilience and wider system benefits as the basis for the selection. A final report is expected in mid-2026. ENR reached out to IESO for comment, but did not receive an immediate response.
IESO’s draft planning materials describe a construction anchored by multiple high-voltage transmission corridors linking major generation sources in the Greater Toronto Area. The draft portfolios include double-circuit 500 kV lines extending approximately 140 km and 190 km from the Bruce Nuclear Complex to central Ontario and the Greater Toronto Area, along with a 60 km 500 kV booster feeding northwest Toronto.
Additional reinforcements include a 50 km double-circuit 500 kV line east of Toronto and an approximately 70 km high-voltage direct current line delivering power directly to the city’s Hearn station, supported by converter facilities.
Plans also call for the expansion of 500/230 kV autotransformer stations and associated substation infrastructure, including switches, buses and reactive equipment to handle increased power flows.
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The IESO’s planning process explains why a definitive list of projects has not yet been released. In its materials, the operator states that the bulk system planning process evaluates multiple infrastructure portfolios in demand scenarios to test system performance under different conditions.
IESO evaluated three transmission portfolios across six performance criteria before selecting Portfolio B, the costliest option at $9.8 billion, at its April 30 South and Central Bulk Plan engagement session. The operator cited portfolio B’s top scores in reliability, operability, resilience and system benefits as the basis for the recommendation. Click to enlarge the image.
Source: IESO
IESO evaluated three transmission portfolios across six performance criteria before selecting Portfolio B, the costliest option at $9.8 billion, at its April 30 South and Central Bulk Plan engagement session. The operator cited portfolio B’s top scores in reliability, operability, resilience and system benefits as the basis for the recommendation.
The scale of construction reflects both increased demand and structural imbalance. More than 1,800 km of transmission development is already planned or underway across the province, according to system planning materials, as it moves toward large-scale grid expansion to accommodate electrification.
Stakeholder submissions indicate that the way forward remains contested. Ontario Power Generation identified a potential of up to 10,000 MW of future generation development at its Wesleyville site and recommended the inclusion of multiple locations in planning studies.
TC Energy supported the development of a 500 kV double-circuit line between the Bruce Nuclear Complex on Lake Huron and Essa, north of Toronto, linked to future generation and storage projects, including the 1,000 MW Ontario Pumped Storage Project.
Hydrostor urged evaluation of bulk connected wireless solutions alongside transmission expansion, while the Energy Workers Union raised concerns that higher-end demand growth scenarios may not be fully captured in planning assumptions, according to IESO’s summary of comments.
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The IESO procurement framework indicates that transmission expansion will continue in parallel with the acquisition of new supplies. Medium- and long-term procurement programs are already underway to secure additional capacity, with planning horizons extending to the 2030s.
The planning record shows a grid under increasing tension. If advanced at the reported scale, the program would be among the largest high-voltage builds in North America, creating a multi-year construction pipeline tied directly to Ontario’s electrification trajectory.
