Load shedding will move from stage 1 to stage 2 from 2pm on Wednesday, Eskom has said.
The power utility said the system was "severely constrained" with demand constantly increasing due to the cold weather.
Stage 2 power cuts are set to continue until 10pm.
In a power update issued on Wednesday, the utility said demand was currently 2 000 MW higher than Tuesday.
"It is forecast that evening peak demand will be much higher than yesterday and the highest so far this year," it said.
This was forcing the utility to burn" large amounts" of diesel, which was depleting stocks.