"The estimates that our best experts gave us three and four years ago are pretty different from those that we're getting right now," the premier told reporters at Georgian College.
"We didn't factor in the single greatest global economic recession in the past 80 years," he said.
That has led to more and more manufacturers and mills in the province shutting down and shaving demand for electricity.
"We've had a massive slowdown in economic growth. I think every year for the last four years we've required less and less electricity – not more, less," said McGuinty.
"Now that will turn around, there will be an increase in demand. We cannot meet all of that through renewables (like hydro, wind, and solar power) and aggressive conservation programs.
"There will be a need for more electricity."
McGuinty said federally owned Atomic Energy of Canada Ltd. is "the front-runner but definitely not the clear-cut winner based on the price" if and when any new nuclear reactors are eventually built.
The premier said his staff has been in touch with Prime Minister Stephen Harper's office in hopes that Ottawa, which is trying to sell AECL's reactor business, will help Ontario pay for new atomic needs.