Stage 2 load shedding will start at 18:00 on Monday evening - not 21:00 as previously planned, Eskom has said in a statement.
"Unit 1 of Kusile and Matimba Unit 5 have just tripped, taking off 1 300MW of capacity adding to the capacity constraints," Eskom said.
Load shedding will continue until 05:00 on Tuesday. More Stage 2 load shedding is planned from 21:00 on Tuesday night to 05:00 on Wednesday morning.
South Africa is currently facing Stage 2 load shedding after several power stations suffered breakdowns over the past week. More than half of Eskom's generation capacity is currently offline.
On Sunday morning, Koeberg Unit 1 tripped as well.
Eskom said this was due to a fault on a feedwater pump, which is on the secondary plant.
"The plant was shut down in accordance with standard plant operating procedures, and all parameters are stable. There are no nuclear safety concerns on the reactor side of the plant, which is ready to be restarted once the feedwater pump fault has been resolved."
Once an investigation into the feedwater pump fault is concluded, Eskom will return the unit to service. It is expected to be during next week.
Due in part to the latest trip, Eskom expects to use emergency reserves "extensively" during the next few days to avoid load shedding.
Emergency generation typically involves using open-cycle gas turbines, which run on diesel. It is a very expensive way to generate power.