Stage 2 load shedding will be implemented from 16:00 on Tuesday afternoon until 05:00 on Wednesday morning.
On Tuesday afternoon, a generation unit at Kusile power station tripped, adding stress to the power system.
A unit each at Matimba and Arnot power stations failed to return to service as previously anticipated.
"These constraints are expected to persist for the rest of the week, which may require the load shedding to be extended," Eskom said in a statement.
Total breakdowns currently amount to 17 933MW, while planned maintenance represents 3 451MW of lost capacity.
Emergency generation reserves are currently "extensively" used to supplement supply, Eskom said.
Over the past 24 hours, Eskom returned a generation unit each at Camden, Kendal and Medupi power stations.
But during the same time, two generation units - one each at the Arnot and Hendrina power stations - tripped.
In addition, another unit at Arnot and a unit at Lethabo power stations were forced to shut down.
Following rolling blackouts over several days last week, load shedding was suspended on Friday evening.
At the time, Eskom chief operating officer Jan Oberholzer expressed confidence that the vote counting process following the elections would not be affected by load shedding.
However, he warned that the power system is still constrained, and some units are "extremely unpredictable and unreliable".
