Load shedding will be increased to stage 4 from 14:00 this afternoon until 22:00 tonight.
From 22:00, South Africa will return to stage 2.
Stage 4 is necessary due to additional breakdowns of a unit at Medupi and two units at Duvha Power Station, together with the high winter demand, Eskom said.
Remaining emergency generation reserves have been depleted and must be restored.
"These emergency reserves are required to respond to further emergencies in order to maintain the stability of the national grid."
Emergency generation typically involves using open-cycle gas turbines, which run on diesel. It is a very expensive way to generate power.
Breakdowns currently total 15 087 MW of capacity, while planned maintenance is 1 273 MW of capacity.
"These capacity constraints will continue for the foreseeable future and all South Africans are urged to reduce their use of electricity," Eskom said.
South Africa was hit by stage 2 load shedding from 10:00 on Wednesday morning due to delays in returning generation units to service at Kusile, Tutuka, Duvha, Kendal and Koeberg power stations.
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.