Support CleanTechnica's work througha Substack subscription,on Patreon, oron Stripe. Help us produce all of thehigh-quality, original content we publish week after weekdespite the challenges of content-scraping AI, antisocial media, inflation, and other hurdles.Ontario needs a lot more electricity. Demand hit 145.6 TWh in 2025, up 4.4% in a year, and IESO’s reference case gets to 250 TWh by 2050. But its own range runs from 207 to 297 TWh. That’s not a rounding error. It’s a completely different buildout depending on which end of the range Ontario actually lands near.
That’s why the most interesting part of IESO’s latest outlook isn’t the 2050 headline. It’s what happens first. After projects already under way are counted, Ontario is short more than 8 TWh of annual energy in 2032 and more than 12 TWh by 2035. The remaining incremental summer capacity gap doesn’t show up until 2035, and it starts at only about 950 MW. Ontario’s first problem is getting enough clean electricity over the year, not finding some enormous block of new peak capacity.
The problem is that “power” isn’t one thing. A wind farm makes a lot of electricity when the wind blows. A battery makes essentially no net annual electricity but can move it into more useful hours. Hydro can provide both energy and flexibility. Transmission can make electricity generated somewhere else useful in Ontario. Demand response can make a peak disappear without generating anything at all. And nuclear can produce enormous amounts of low-carbon electricity and dependable capacity for decades. Pretending they are all selling the same product is a good way to buy the wrong mix.
Ontario has finally started buying wind and solar again after more than a decade without a major new-build renewable procurement. The first LT2 energy window signed 1,115 MW of projects expected to produce about 2.37 TWh a year. IESO said the initially selected projects came in 21% below pricing from the previous comparable large-scale renewable procurement. That doesn’t mean wind and solar solve everything. It means Ontario can add a meaningful amount of clean electricity in a few years without deciding today what its entire 2045 generating fleet has to look like.
Ontario is finally buying the stuff that lets it use more wind and solar without leaning so hard on gas, too. The latest LT2 capacity window added another 640 MW of batteries, taking contracted storage above 3.5 GW by 2030. More importantly, the price is moving in the right direction. IESO says this round was 36% cheaper than its expedited first long-term battery procurement and 16% cheaper than its first regular one. That’s what repeat procurement is supposed to do: reveal today’s prices instead of asking planners to guess what storage will cost twenty years from now.
That matters because Ontario has backed itself into much greater gas dependence. Transmission-connected gas and oil generation rose from 9.7 TWh in 2020 to 31.4 TWh in 2025, while nuclear output fell during refurbishments and outages. Gas went from 7% to 19.3% of transmission-connected generation in five years. Some of that is simply more energy demand, but gas is also doing a lot of the balancing work around Ontario’s nuclear-heavy system: reserves, load following and frequency response.
So replacing gas isn’t just a matter of adding another big source of low-carbon annual generation. Ontario also has to replace the jobs gas does hour by hour. Batteries, hydro, transmission, interties and controllable demand are increasingly capable of doing that. None is magic and none eliminates the others, but they are exactly the kinds of resources Ontario needs if it wants gas pushed back toward the margins instead of remaining the thing that keeps a low-carbon grid operable.
And this isn’t an argument for shutting reactors. Ontario’s existing nuclear fleet is one of its most valuable low-carbon assets, and the recent refurbishment record is genuinely good. Darlington’s refurbishment finished ahead of the overall schedule and below budget. Bruce Unit 3 came back seven months early and under budget. Extending productive reactors at sites that already have transmission, trained workforces and operating organizations is a very different proposition from deciding how many brand-new reactors Ontario should order for the 2040s.
The Darlington SMRs are a much riskier proposition than the refurbishment program. Ontario is not restarting a familiar CANDU construction line. It is building a GE Hitachi BWRX-300 boiling-water reactor, a design Ontario has never operated, with a different fuel cycle and supply chain from the CANDU fleet. The province also hasn’t built a new reactor in decades, so the institutional capability that delivered the original fleet cannot simply be assumed to be sitting on the shelf waiting to be restarted. The first four units may produce useful evidence, but they should be treated as a heavily backed first-of-a-kind nuclear program whose cost and schedule performance have to be demonstrated, not as evidence that SMRs have already solved nuclear’s construction problems.
The next nuclear decisions are where the money gets even more serious, and Ontario should not assume that success at Darlington is the base case. The BWRX-300 program first has to show that Ontario can build an unfamiliar reactor design repeatedly, control first-of-a-kind costs and rebuild a nuclear construction supply chain that has been dormant for decades. Wesleyville could eventually hold as much as 10 GW of generation, and Bruce C is moving through pre-development. Keeping those options alive makes sense when IESO’s 2050 demand cases differ by about 90 TWh. Committing now to build all of that capacity does not.
If Ontario ends up near IESO’s high-demand case and a large block of clean generation is still missing after all of that, big new nuclear could make sense. If demand lands closer to the reference or low cases while renewables, storage and flexible demand keep getting cheaper, the economically sensible nuclear build could be non-existent. Ontario doesn’t have to guess which future wins in 2026. So build the fast stuff first, keep refurbishing reactors that are delivering value, learn from the SMRs already under construction, and keep the bigger nuclear sites available. Then make the really expensive, really long-lived decisions with better information.
Ontario’s Conservatives deserve criticism for spending years turning renewables into a political wedge and wrecking much of the development pipeline they now need to rebuild. But replacing one politically predetermined answer with another would be no smarter. The question isn’t whether Ontario is a “nuclear province” or a “renewables province.” The question is what combination gets enough clean TWh onto the system, replaces gas flexibility and leaves the smallest expensive residual problem for new firm generation to solve.
For the full analysis of Ontario’s gas dependence, grid operability, renewables, storage, flexible demand and nuclear options, read ** Ontario Needs New Generation And Flexibility. Nuclear Should Compete For What Remains** at TFIE Strategy Briefing. Sign up for
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