It had initially announced a 10-hour rationing that kicked in late Saturday to fix a broken conveyor belt at one of the major coal-fired plants, but on Sunday it extended the outages by another day.
Known as "load shedding", the cuts are implemented to prevent a collapse of the electricity grid. Last year they were implemented in February, October and again in December when Eskom rationed 6,000 megawatts from the national power grid - plunging the country into its worst darkness in many years.
"The system remains constrained and vulnerable, and as such load shedding .... will unfortunately have to continue from 08:00 (0600 GMT) this morning until 05:00 tomorrow morning (Monday)," Eskom said in a Sunday statement.
"Owing to inadequate maintenance over a number of years, the system remains vulnerable to unplanned outages," it warned.
The latest power cuts also come the weekend before Eskom's newly appointed CEO is expected to report for duty. Andre de Ruyter is expected to take over the reins at Eskom on Monday, 10 days ahead of his initially scheduled starting date of January 15, reportedly after cabinet intervention.
De Ruyter will oversee a plan to turn around the utility which has long struggled to produce enough power due to poorly maintained coal-fired power stations as well as decades of mismanagement and alleged corruption.
A conveyor belt failed on Saturday at the new highly-touted Medupi plant which, along with the Kusile site, "were badly designed and badly constructed", according to the Public Enterprises Minister Pravin Gordhan. Eskom has amassed 450 billion rand ($30 billion) in debt, which economists warn is a major threat to South Africa's overall economy.