Eskom has warned of Stage 2 load shedding from 4pm this afternoon until 23:00 due to a shortage of capacity.
It tweeted the warning from its official account on Thursday afternoon.
Load shedding was necessary to prevent "total collapse" of the electricity system, the power utility said in a follow-up statement. The system is currently severely constrained, it added.
According to the statement, unplanned breakdowns are currently at 12 300 MW.
"We unreservedly apologise to South Africans for the impact this may have on them [...]," Eskom said.
The power utility recently said no load shedding was planned until March, although load shedding could be implemented if absolutely necessary, Fin24 reported.