The power generation system is "severely constrained" due to two generation units tripping on Tuesday, Eskom said in a statement.
The power utility said it had lost a unit each at Medupi and Tutuka power stations, amounting to a total loss of about 1 300MW. The "constrained power situation" could persist through the weekend, it warned.
It appealed to the public to use electricity sparingly.
This was the second warning issued by Eskom on Tuesday, after the power utility earlier in the afternoon said it would be implementing "load reduction" - a form of localised rolling blackouts - in parts of Gauteng.
Gauteng, which would be experiencing load reduction for a full five hours on Tuesday evening, has been subject to periods of these localised rolling blackouts since June, which Eskom has attributed to a high incidence of illegal connections, meter bypasses and vandalism.
South Africa has been under lockdown since March, with many of those who can opting to work from home. Eskom initially said the lockdown had dramatically reduced power demand.
However, Eskom CEO Andre de Ruyter has also warned that the risk of load shedding remains.
In a recent interview with Fin24, he noted that catching up on the maintenance backlog would take from July until approximately the end of August 2021.
Long-term, intensive maintenance was planned from 1 July this year, he said.
The power utility also announced that it will "curtail" electricity supply to Magareng local municipality from tomorrow, the 8th of July because of non-payment. Eskom was planning to effect daily power interruptions to four other local municipalities in the Northern Cape – Kamiesberg, Khai-Ma, Richtersveld and Tsantsabane but they entered reached some form of agreement on how each would try and pay off its outstanding debt.
"Unfortunately, no satisfactory payment agreement has been reached with the Magareng Local Municipality. Therefore, electricity supply services to Magareng will be curtailed starting Wednesday, 08 July 2020. The Municipality will have to endure daily power restrictions until the payment breach has been remedied by the Municipality," said Eskom.
It added that electricity users in Warrenton and Warrenvale will also be affected by these interruptions.