Eskom announced on Monday afternoon that load shedding would move from Stage 2 to Stage 4 from 13:00.
The rotational power cuts are expected to continue until 22:00.
The power utility, which instituted Stage 2 load shedding at 09:00 on Monday, said it had been forced to move to Stage 4 after it unexpectedly lost six additional generating units.
It did not say why the units were lost.
Stage 2 allows for up to 2 000 MW to be cut from the national grid, while Stage 4 allows for 4 000 MW to be cut.
On Sunday the power utility instituted load shedding for the first time since December. It came three days after President Cyril Ramaphosa announced that Eskom would be split into three in his State of the Nation Address.
The load shedding websites of some municipalities returned server busy errors on Monday afternoon due to the number of users trying to find information. Requests were generally resolved within a few minutes.
Ratings agency Moody's, meanwhile, said in a research report on Monday that splitting Eskom into three entities may make the utility's cost structure more transparent, but the split does little to address the debt-laden power utility's financial challenges.
The agency published a research note in reaction to President Cyril Ramaphosa's State of the Nation Address on Thursday, where he announced that the power utility would be split into three separate entities. The entities would all still fall under Eskom.