More than 4,000 residents and businesses lost power just after 8:30 a.m., mostly in Butler and Butler Township.
Several hundred power customers also were out in Penn Township with fewer outages in Center and Jefferson townships and East Butler.
The West Penn Power website said power is to be restored by 11 a.m., although some customers were restored earlier.
Power company spokesman Rob Lombardo said the cause of the outages still is under investigation.
The outage closed access to the county courthouse and government center for a time this morning. However, access was restored by 9:30 a.m.