Rocky Mountain Power informed the Public Service Commission of the pending closure in October.
Officials say the plant has been relying on short-term energy contracts and lower power prices have caused it to lose money. The Hardin Generating Station employs 30 people and buys coal from the Absaloka mine on the nearby Crow Indian Reservation.
The plant owes more than $2 million to Big Horn County in delinquent taxes and penalties. Hardin issued $12 million in bonds to build infrastructure supporting the power plant. Payments on the bonds are in default because the plant isn't paying its taxes.