Previously, the most discussed closure date for Units 1 and 2 was December 2022. That date was tied to a July 2016 legal settlement in which Colstrip owners agreed to shutter the units "no later than" 2022. The settlement was between the unit owners, the Sierra Club and The Montana Environmental Information Center. The lawsuit centered on air pollution.

Units 1 and 2 were built in the mid-1970s. The economics of the oldest units have been troubled for years. In 2016, Talen gave the other five owners of the power plant two years' notice that it would cease operating the plant by mid-2018. Talen at the time said it was losing millions of dollars per quarter at Colstrip.

Later, Talen reconsidered and continued to operate the plant. The power plant's other owners — Oregon utilities PacifiCorp and Portland General Electric; Washington utilities Puget Sound Energy and Avista Corp.; and South Dakota-based NorthWestern Energy — weren't interested in running the power plant's daily operations.

The closure will speed development of new energy sources, particularly for Puget Sound Energy, which owns half the output from Units 1 and 2. The largest Colstrip stakeholder has been making plans for replacement energy, including contracts with proposed renewable energy projects in Montana. But those plans didn't assume closure during the next six months.