Stage 2 load-shedding will be implemented from 17:00 on Tuesday due to delays bringing generating units back online, Eskom said in a statement.
Eskom earlier warned that the power system was severely constrained.
Breakdowns currently total 15 498 MW of capacity, while another 1 273 MW of capacity is unavailable due to planned maintenance.
South Africa has been hit by another bout of load-shedding over the past month. Most recently, breakdowns of generating units at the Majuba and Arnot power stations hit the power supply, as did delays in returning units to service at Arnot and Tutuka power stations.
In addition, unit 1 of the Koeberg power station has been offline since January for maintenance work. It was supposed to return to service last month, but this still hasn't happened.
Eskom on Friday suspended Koeberg's general manager Velaphi Ntuli over performance issues related to the delayed return to service.