The SkyPower Group is launching the lawsuit over changes Premier Dalton McGuinty's government made to the energy program, including cutting the price it will pay for solar energy.
SkyPower had applied to build more than 100 solar projects in Ontario by this past April when the province announced the price changes and new criteria for approving projects.
"The government changed the rules of the game," Toronto lawyer Tim Gilbert said. "We say simply that's unfair. Most people would agree it's not fair to change the rules of a process after you've relied upon it."
SkyPower says the changes are unreasonable and would kill many of its projects.
The company wants the courts to strike down the changes and wants its projects re-considered under the old rules.
Without that, it's looking for $100 million in damages.
Energy Minister Chris Bentley said in a statement that the government will vigorously defend the lawsuit.
Bentley points out the green-energy rules had been under review.
The government was already being sued for putting a moratorium last year on offshore wind power projects.