HELENA, Mont. -

The state Public Service Commission has ordered NorthWestern Energy to refund an estimated $2 million to $3 million to customers after the commission found the utility overestimated how much revenue it lost from energy-conservation programs.
    
Tuesday's decision focused on how much revenue NorthWestern reported losing from 2006 through 2011 from promoting the use of energy-saving light bulbs.  Customers' electric rates already cover the cost of the promotions, but NorthWestern also seeks to recoup the cost of electricity not used because of higher compact fluorescent light bulb use.
    
The PSC found a study overstated how much electricity wasn't used and said a portion of the reimbursements should be refunded to customers through a rate credit.