BILLINGS, Mont. — Northwestern Energy will seek 55 megawatts of additional power-generating capacity by 2019 and possibly ten times that much over the next decade, to ensure it can satisfy customer demands as the availability of coal-fired electricity ebbs, representatives of the utility said Monday.

The move comes as large coal-fired power plants across the Pacific Northwest — including the Colstrip Steam Electric Station in eastern Montana — have announced plans to scale back or end operations in coming years.

NorthWestern is a partial owner of Colstrip and serves more than 700,000 customers in Montana, South Dakota and Nebraska.

Environmental activists protested in front of the utility’s Montana headquarters in Butte on Monday, urging NorthWestern to move away from fossil fuels and use more renewable wind and solar energy.

NorthWestern Vice President John Hines said the utility will consider all sources of electricity, including natural gas and renewable energy supplies such as wind and solar. But company officials said gas plants have advantages over renewables because they can be switched on and off quickly during periods of peak electricity use.

The company already has enough so-called baseload power-generating capacity to meet its typical demands. The additional power is needed to cover those hours when consumer use spikes, Hines said.

NorthWestern projects it will need 500 to 600 megawatts of additional power to meet customer needs over the next decade.

The utility plans to issue a formal request for proposals to meet its most immediate power needs by early next year. The Billings area has been identified by Northwestern as the least costly location for a new natural gas plant.

Activists from the group 350 Montana said documents submitted by the utility to state regulators show NorthWestern is laying the groundwork to build more than a dozen gas plants in coming years at a cost of $1.3 billion.

The group contends that natural gas is just as bad as coal when it comes to climate change because of the large amounts of gas that escape into the atmosphere from leaking pipelines and other accidental sources.

“It’s not a climate solution in any way,” said Bill McKibben, co-founder of “When you build these plants, they are supposed to work 40, 50 years. It locks us in way past the middle of the century to dirty fuel.”

Company officials said a final decision on what kind of plants to build and how many would be made at a later date. They described the $1.3 billion cost estimate as a “placeholder” figure that’s subject to change.

“At this point with the low price of natural gas, it would likely be a consideration. But all of that is way short of a proposal,” NorthWestern spokesman Butch Larcombe said.