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It shows that last year 16 per cent of the electricity in Ontario's grid was produced by natural gas, making it 84 per cent emissions free – down from 87 per cent the year before and down from a high of 96 per cent in 2017.

The last time Ontario's electricity was produced with at least 16 per cent of emitting power was in 2012, when the province still had coal-fired generation. 

Nuclear led the way in 2024, with 51 per cent of Ontario's electricity generated by those plants, and about 24 per cent came from hydro power.

The percentage of natural gas generation in Ontario's electricity system has been increasing over the past several years. The IESO says gas provides more flexibility than many other sources, and more is being used while some nuclear units undergo refurbishments so the system remains stable.

"Outages to nuclear generation from this work combined with reduced supply from imports also resulted in gas generation’s higher contribution to overall output in 2024," the IESO said in its report.

Aliénor Rougeot, climate and energy senior program manager at Environmental Defence, said the need to resort to gas to fill in the gaps was preventable if Ontario had acted sooner to boost renewable energy and battery storage.

"I think this is one of those key moments that is extremely upsetting and yet super predictable, which is that the energy planning and the poor decisions that get made five, eight years prior to that now are starting to show up in those supply mixes," she said.

Premier Doug Ford cancelled 750 renewable energy contracts shortly after his Progressive Conservatives formed government in 2018, after the former Liberal government had faced widespread anger over long-term contracts with clean power producers at above-market rates.

The IESO had been set in 2023 to seek out non-emitting electricity generation to bring more capacity into the system but after Energy Minister Stephen Lecce came into the portfolio in 2024, he announced the procurement would be "technology agnostic." Critics said that not only opens the door to natural gas but may favour bringing more of it online.

Lecce said the rise in non-emitting electricity generation is "entirely predictable" due to the ongoing nuclear refurbishments.

"Having said that, we're actually going to have a greener, cleaner grid, getting down to below 99 per cent non-emitting by 2050," he said.

"So this is part of the journey as we take our non-emitting nuclear units off the grid, because they need to get refurbished for another 40, 50 years."

The government often touts Ontario's clean electricity grid as a top selling point when attracting businesses and investment to the province, and Green Party Leader Mike Schreiner said he's not sure that at 84 per cent clean it is as attractive as it once was.

"I'm deeply concerned that our grid has gotten 12 per cent dirtier over the last few years, especially when the government itself says that one of the competitive advantages Ontario has in attracting global capital investment is our clean grid," he said. 

"The Ford government is losing that advantage at a time we need to be attracting capital more now than ever."

Lecce said the electricity grid is still a good selling feature and will only get better.

"When we're getting to near 99 per cent, it's an incredible achievement that no industrialized economy can point to," he said. 

"We're very proud of the fact that we have one of the cleanest grids today. We'll have an even cleaner grid tomorrow as we get our nuclear fleets back."

In addition to ongoing and planned refurbishments of units at the province's large-scale nuclear plants, construction on the first of four small modular reactors is set to begin this year.

The $21-billion project for the four reactors is expected to produce enough power for 1.2 million homes.

Stephen Thomas, clean energy manager at the David Suzuki Foundation, said that small modular reactors are one of the most expensive generation options. But natural gas can also be expensive, and the province should be looking more at renewables, he said.

"We think wind and solar are ready for prime time," he said. 

"As we look to the future, we only see the cost of wind, solar and storage coming down over time, and we see the opposite for natural gas. The cost of natural gas is hugely volatile, and it's hard to depend on what the spot price of natural gas might be in 10 years or 20 years." 

This report by ¹ú²úÓÕ»ó¸£Àû was first published May 14, 2025.