Thousands of homes, shops and offices were left without power after thieves stole a copper earthing cable from an electricity sub-station.
About 20,400 premises were affected in the Dudley and Tipton areas of the Black Country.
The sub-station in Dudley, where the incident happened on Thursday morning, was shut down for safety reasons.
Central Networks said 8,000 had power restored within an hour and the rest were back on by early afternoon.
Supplies went off at about 1120 GMT, a spokesman said.
He said: "We decided to remove power to 20,400 customers after the break-in. We hope customers understand why we had to do that."
Engineers have been replacing the cabling and switching customers to other power circuits where possible.