City Power said on Thursday load-shedding was straining its resources, with some substations unable to return to normal after power is restored.
This left customers without power for longer.
“The past two days, Tuesday and Wednesday, were the worst, with most of our substations failing to switch on after restoration, with Randburg the hardest hit,” City Power said in a statement.
It said the recent bout of load-shedding, announced by Eskom this week and expected to last until Sunday night, comes as City Power is already overstretched by an increase in demand for winter, while battling rampant cable theft and illegal connections.
On Thursday afternoon, City Power teams were struggling with some of the substations that were still off.