Power was restored by 4 p.m. Tuesday to about 338 Ameren Illinois customers in Alton and Godfrey that had been without electricity since approximately 11 a.m.
Only one commercial customer in Alton remained without power, Ameren spokesman Brian Bretsch said.
The cause of the outage, which initially affected about 600 customers, was equipment failure in a pad-mount transformer, he said.
"For the one remaining commercial customer, we actually are bringing in a new transformer," he said. "When that arrives on site, we'll have a three-man crew who will continue working on that throughout the evening until we can completely and safely restore that customer."
Bretsch said anyone experiencing a power outage should call Ameren Illinois at (800) 755-5000. Outage information is available at https://www.ameren.com/illinois/outage-center.