Montana's largest electricity provider has released a draft of its plan to expand its power generation capacity by 800 megawatts by 2025.

The draft plan released Tuesday by NorthWestern Energy says natural gas power would be the cheapest way to expand, but the company will solicit competitive bids from a wide variety of technologies.

Brian Fadie of the Montana Environmental Information Center says he is concerned that company officials have already decided to build 800 megawatts worth of new natural gas plants.

The plan says the company also will consider "opportunity resources," or existing power generation that becomes available for purchase. A bill is pending before the Montana Legislature that would allow NorthWestern to buy out its co-owners of the coal-fired Colstrip power plant for $1.

The public has 60 days to comment on the plan.