Thousands of homes and businesses along the Hi-Line are without electricity because of a failure at a substation near Havre.
NorthWestern Energy spokesman Butch Larcombe says 17,000 customers initially were affected in the outage from Chester to Glasgow and south to Big Sandy.
Electricity has been restored to Chinook and Glasgow, but thousands remain without power in Havre and to the west.
It was 33 degrees in Havre Friday.
Larcombe says an equipment failure at the substation caused the outage. He says the utility does not have a firm estimate of when power will be restored.