NorthWestern Energy has signed an asset purchase agreement with Spion Kop Wind to buy its 40MW wind project being constructed in Basin County in the US state of Montana.

The project is in line with the state legislation and will be placed into utility rate-base for electric retail customers of the company.

Construction on the project will begin following approval from the Montana Public Service Commission on or before April 2012.

Commercial operation is set to begin by the end of 2012.

NorthWestern has previously executed a memorandum of understanding with Compass to buy a 24MW wind farm in central Montana.