Ontario, Canada’s most populous province and its economic powerhouse, is embarking on an unprecedented energy transformation. The Independent Electricity System Operator’s (IESO) 2025 Annual Planning Outlook (APO) projects that the province’s electricity demand will surge 75 per cent by 2050, reaching 262 TWh annually. This is equivalent to adding four-and-a-half cities the size of Toronto (Ontario’s capital city) to the grid. Released in June 2025, “Energy for Generations” represents the province’s first comprehensive Integrated Energy Plan (IEP), marking a departure from traditional, isolated approaches by coordinating electricity, natural gas, hydrogen and other energy sources into a unified strategy. It will guide Ontario’s energy planners and regulators based on four core principles of affordability, security, reliability and clean energy.

This transformation reflects a fundamental shift from managing electricity surplus to actively building new capacity for economic growth. The province plans to expand nuclear capacity by up to 17.8 GW by 2050, while investing CAD4.7 billion in hydroelectric refurbishments. Ontario’s integrated planning philosophy recognises that energy drivers are being reshuffled; industrial customers are increasingly choosing clean electricity over fossil fuels, while residential customers are becoming energy producers through rooftop solar and battery storage. The plan acknowledges that customers are best positioned to decide which energy solutions work for them, based on their needs, preferences and budgets, creating a more complex yet flexible energy ecosystem.

This transformation is underpinned by institutional changes through the recently proposed Bill 40, which formally adds “economic growth” to the mandates of both the Ontario Energy Board (OEB) and the IESO, requiring these agencies to consider job creation and economic development in their decision-making. Ontario has announced a CAD160 million contribution from the Clean Electricity Fund alongside a new Transmission Fund to support transmission and clean energy initiatives. The plan also includes increased annual funding of CAD25 million for the Indigenous Energy Support Program (IESP) for capacity building and equity partnerships.

The transmission network faces equally dramatic changes, with the province planning significant grid modernisation to accommodate bidirectional power flows as distributed energy resources reshape how electricity moves across the system. New regulatory frameworks are being developed to streamline transmission development, while major infrastructure investments will connect emerging clean generation sources to Ontario’s growing industrial and urban demand centres. To meet these unprecedented challenges, Ontario’s IEP charts a comprehensive transformation across three critical dimensions: demand forecasting, supply generation and transmission infrastructure. Each pillar presents unique challenges and opportunities that will reshape the province’s energy landscape over the next quarter-century.

This feature focuses on the power-related components of Ontario’s IEP, examining the province’s strategy for meeting unprecedented demand growth through nuclear expansion, renewable integration and massive transmission infrastructure development.

Understanding forecasts: Planning for a range of futures

Ontario’s energy planners are using a scenario-based approach that prepares for multiple possible futures. The IESO’s base forecast projects demand to reach 262 TWh by 2050, but the most aggressive “Pathways to Decarbonization (P2D)” scenario envisions demand climbing to 325 TWh by 2050, nearly 25 per cent higher than the base case.

Source: Energy for Generations, Government of Ontario (June 2025)

This surge is driven by several converging forces. Transportation electrification is expected to become the single largest contributor, with electricity consumption from electric vehicles (EVs) alone forecast to reach 20 TWh by 2035. Reindustrialisation is projected to increase electricity use by 58 per cent by 2035. Data centres supporting artificial intelligence (AI) and cloud computing represent 13 per cent of the new demand growth, while population increase of 4.2 million people by 2051 contribute to residential and commercial loads.

Supply transformation: From today’s clean grid to tomorrow’s nuclear renaissance

Ontario’s electricity system already boasts one of the world’s cleanest grids, with nuclear and hydroelectric power providing nearly 50 per cent of the current generation. The province’s existing supply mix reflects decades of strategic investment in low-carbon baseload power.

Source: Energy for Generations, Government of Ontario (June 2025)

Ontario’s current 38.8 GW generation mix comprises nuclear (25 per cent), natural gas (28 per cent), hydroelectric (24 per cent), wind (14 per cent), solar (7 per cent) and smaller contributions at about 1 per cent from bioenergy and storage. However, meeting the projected 75 per cent demand increase will require a dramatic expansion of the supply fleet, fundamentally reshaping Ontario’s generation landscape over the next 25 years.

Note: This figure shows an IESO projection from the 2025 APO of how the sources of electricity produced in Ontario could evolve over the planning period to meet demand in a cost-effective manner.
Source: Energy for Generations, Government of Ontario (June 2025).

The transformation will unfold in phases, with nuclear capacity expected to grow substantially from the current 65.4 TWh to potentially 200 TWh by 2050, cementing its role as the backbone of Ontario’s clean electricity system. This expansion includes ongoing refurbishments at Bruce and Pickering stations, four new small modular reactors aggregating 1.2 GW at Darlington, which are expected to come online between 2030-35 and potential large-scale nuclear developments at Bruce C (4.8 GW) and Wesleyville (up to 10 GW).

Natural gas will play a complex transitional role, initially increasing during the 2020s and early 2030s as nuclear units undergo scheduled refurbishments, before declining significantly as new non-emitting capacity comes online. While natural gas generation may temporarily rise to support reliability during nuclear outages, its long-term trajectory points toward a significantly reduced role by 2050, dropping from 31.3 TWh currently to potentially just 1.6 TWh annually.

Hydroelectric generation will maintain its crucial position, with Ontario Power Generation (OPG) investing CAD4.7 billion in refurbishment and expansion projects to secure over 5 GW of capacity. The IESO has identified 3-4 GW of untapped hydroelectric potential in northern Ontario, which could support economic reconciliation with Indigenous communities, while unlocking critical minerals development.

Renewable resources, including wind and solar, will experience significant growth in the medium term, supported by competitive procurements and energy storage integration. However, their contribution may moderate over time as nuclear capacity expands dramatically, with non-hydro renewables potentially declining from peak levels of around 50 TWh in the late 2030s to approximately 23 TWh by 2050 as system needs evolve.

Transmission infrastructure: Connecting clean power to growing demand

The massive expansion of Ontario’s electricity demand and supply will only succeed if complemented by an equally ambitious transmission buildout to move power from generation sites to load centres. The grid expansion is anchored by Hydro One Networks Inc., Ontario’s largest electricity transmission and distribution utility, which owns and operates over 30,000 km of transmission lines serving 1.5 million customers. The province is embarking on one of the largest transmission infrastructure programmes in Canadian history, with several major projects spanning from northern mining regions to the densely populated Greater Toronto Area (GTA).

Ontario’s IESP, recently expanded with a CAD10 million annual increase to CAD25 million, supports Indigenous communities in energy planning, development and skills training, while promoting energy knowledge and awareness. This programme complements the province’s pioneering 50:50 equity partnership model with Indigenous communities, which provides economic participation and ownership stakes in transmission projects, advancing reconciliation, while ensuring local benefits from infrastructure investments in traditional territories.

Strengthening the North-South Corridor

The centrepiece involves reinforcing the critical north-south electricity backbone. This includes a new single-circuit (S/C) 500 kV line between Essa substation (Barrie) and Hanmer substation (Sudbury), targeted for service in 2032 and the Orangeville to Barrie reconductoring project to upgrade existing 230 kV lines with advanced conductors.

Powering GTA growth

The GTA’s expanding electricity needs require targeted transmission investments. The new 500 kV Bowmanville to GTA double-circuit (D/C) line, expected to be in-service in the early 2030s, will connect OPG’s new SMR units 2, 3 and 4 at Darlington, while enhancing electricity delivery to the downtown core. Meanwhile, the Manby to Riverside Junction reconductoring project will upgrade 115 kV circuits to support growing demand in the Western GTA.

Southwest Ontario industrial expansion

Southwest Ontario’s rapid industrial growth, driven by EV and battery manufacturing investments, requires substantial transmission reinforcement. The recently completed Chatham to Lakeshore Line achieved commercial operation in January 2025. The St Clair Line will provide an additional 230 kV capacity between Lambton and Chatham by 2028, while the Longwood to Lakeshore system will deliver 500 kV service in phases through 2030.

Planning for growth in eastern Ontario

Eastern Ontario, stretching from Peterborough to the Ottawa area, is experiencing significant demand growth driven by population increases, new residential development, and expanded electrification. The Durham–Kawartha line between Dobbin substation (Peterborough) and Clarington substation, expected in-service by 2029, represents the cornerstone of eastern Ontario’s expansion. Ottawa area reinforcement is being advanced to support the rapidly growing National Capital Region, while an Eastern Ontario system study is examining transmission needs across Ottawa, Kingston, Belleville and Peterborough.

Unlocking northern opportunities

Expanding transmission in the northern region, which is central to Ontario’s long-term electricity and economic strategy, will support critical minerals development, enable new hydroelectric opportunities, and advance reconciliation with Indigenous communities. Mining projects across the North are set to become major electricity consumers, while dozens of remote, diesel-reliant First Nations stand to benefit from long-term grid connections. The IESO has identified 3 GW-4 GW of untapped hydro potential in northern Ontario, which could help meet future demand, while creating local benefits.

The Greenstone area

The 230 kV Greenstone line, a significant project in northern Ontario, will connect the Longlac substation near Geraldton to the Nipigon generation station by 2032. Developed through Hydro One’s Shared Prosperity Agreement with the Aroland First Nation and other Indigenous partners; the line will provide reliable electricity to support existing mines and unlock new mineral development in the region. It will also serve as a foundation for future grid extensions to remote First Nations that continue to depend on costly and polluting diesel generation. The project demonstrates how transmission expansion can advance economic development, Indigenous participation and environmental goals, simultaneously.

Advancing transmission in the Red Lake area

In northwestern Ontario, the Red Lake area is emerging as another critical mineral hub requiring new electricity infrastructure. The IESO is finalising an addendum to the 2023 Northwest Integrated Regional Resource Plan, expected by late summer 2025, to assess the region’s future grid requirements. The Red Lake initiative reflects Ontario’s proactive approach: planning transmission ahead of demand so that electricity does not become a barrier to mining investment, Indigenous economic participation, or the province’s role in supplying critical minerals for the global energy transition.

Strategic corridor planning and future development

Beyond immediate construction projects, Ontario is taking a forward-looking approach to transmission planning through strategic corridor preservation and specialised regional studies. Recognising that future transmission needs will be constrained by land availability in high-growth areas, Ontario is proactively identifying, planning and protecting strategic transmission corridors. The province is advancing three major corridor studies in partnership with the IESO and Indigenous communities:

  • The Parkway Belt West Corridor Study will examine existing GTA corridors, including current Parkway Belt West Plan lands, to identify locations for new high voltage lines (230 kV and/or 500 kV) and transformer stations. This corridor would facilitate electricity movement into and throughout the GTA, including new supply from nuclear generation, while accommodating lower voltage infrastructure for new residential, manufacturing and commercial development.
  • The Barrie to Markham Corridor Study will identify and preserve land for a future 230 kV line, ensuring availability for the southern phase of the north-south connection. This corridor will enable power delivery from northern generation facilities into some of the highest growth GTA areas.
  • The Northwest GTA Corridor Study will revise the narrowed area of interest adjacent to the future Highway 413, accommodating both 230 kV and 500 kV infrastructure to support load growth in York, Peel and Halton regions, while enabling supply from new generation resources to reach high-growth GTA areas.

By preserving options for transmission expansion in rapidly developing regions, Ontario can build necessary infrastructure when and where needed to support electrification, housing development and economic growth, without compromising system integrity or facing prohibitive land acquisition costs.

Ontario’s interties: Strengthening continental energy security

Ontario’s position as a continental energy hub is reinforced by its strategic transmission interconnections (interties) with neighbouring jurisdictions. Ontario currently maintains interconnections with five neighbouring jurisdictions, representing a total nominal capacity of about 6 GW, which supports system reliability, while creating revenue opportunities through electricity sales to neighbouring jurisdictions.

–Canadian Interties

Ontario’s connections with other Canadian provinces support interprovincial electricity trade and strengthen national energy security:

  • Ontario–Quebec Intertie represents the province’s most significant interprovincial connection, with existing transmission lines, enabling substantial electricity trade between the two provinces. This connection allows Ontario to access Quebec’s abundant hydroelectric resources during peak demand periods, while providing Quebec access to Ontario’s nuclear baseload capacity. The two provincial governments continue to explore opportunities to expand trade through enhanced transmission capacity.
  • Ontario–Manitoba Intertie provides access to Manitoba’s hydroelectric resources and creates opportunities for seasonal electricity exchanges. This connection supports system reliability for both provinces and enables cost-effective resource sharing, particularly during periods of high demand or supply constraints.

–US Interties

Ontario’s connections with the US provide access to broader North American electricity markets and enhance continental energy security. This includes interties with New York, which comprises multiple transmission lines to New York State’s electricity grid; with Michigan linking Ontario’s grid to the Midwest Independent System Operator (MISO) region; and with Minnesota providing an additional connection to the MISO region, enhancing system flexibility and creating opportunities for electricity trade with the US Midwest.

Ontario continues to evaluate opportunities to enhance its interties as part of its broader transmission expansion strategy, recognising their importance to Ontario’s role as a continental clean energy supplier.

Note: All the upcoming projects are developed by Hydro One; *The line will be initially energised at 230 kV.
Source: Energy for Generations, Government of Ontario (June 2025); Global Transmission Report.

Competitive transmitter selection framework

Recognising the scale of required transmission development, Ontario is introducing competition to the traditionally utility-dominated sector. The IESO is developing a Transmitter Selection Framework, starting with a Transmitter Registry in Q3 2025 to pre-qualify capable transmitters and enable efficient procurement.

The framework aims to lower costs through competition, while embedding pathways for Indigenous leadership and equity participation. Projects will be assessed against eligibility criteria such as sufficient lead time, cost and voltage thresholds. Some urgent or complex projects may still require direct designation, but competition will be maximised where timelines allow. This shift toward market-driven transmission development is expected to reduce ratepayer costs, encourage innovation and broaden Indigenous participation.

Conclusion

Ontario’s transmission infrastructure expansion represents one of the largest buildouts in Canadian history, designed to connect clean generation resources to growing demand centres across every region of the province. From the critical north-south backbone to strategic interties with Quebec, Manitoba and three US states, this comprehensive programme will enable Ontario to meet its projected demand increase by 2050, while positioning the province as a continental clean energy hub. Through Indigenous partnerships, competitive procurement frameworks and coordinated regional planning, Ontario is building the transmission foundation necessary to support electrification, economic growth and energy security for generations to come.