Several suburbs in and around Johannesburg were hit by a widespread power outage just before 2pm on Friday.
City Power spokesperson Isaac Mangena said: "This is due to the trip on an Eskom line in the north that was undergoing planned maintenance earlier today which affected several of our substations - at least 20 of them with each servicing at least 10 suburbs.
"So it's a huge area. We are working closely with Eskom to try to restore power," said Mangena.
He could not confirm when the power would be back.
"We apologise for the inconvenience."
Business and homeowners vented their frustration on social media, directing a stream of tweets at City Power.