No load shedding is expected on Sunday, Koeberg said in a statement.
"We have successfully returned Koeberg Unit One to service after a planned outage, which has added an additional 900MW to the system."
After being out of commission for planned maintenance for months, Unit 1 was switched on Monday morning this week and slowly started feeding electricity into the national grid.
At full capacity, it should deliver at least 970MW.
Owning to inadequate maintenance over a number of years, the system remains vulnerable with unplanned outages at 13,859MW as at 06:30 on Sunday morning, the utility said.
"We will give South Africans an update about our prognosis for the coming working week later today," Eskom said.