Johannesburg - Eskom will implement stage two black-outs from 8am on Saturday, the power utility said.
"The electricity supply system is very vulnerable due to a shortage of generation capacity as several units are currently out of service due to planned and unplanned outages," Eskom said in a statement.
"We will use the weekend to build up diesel stocks and dam water levels for open-cycle gas turbines and pumped storage schemes respectively."
The power cuts were expected to continue until 10pm on Saturday.
Eskom said it would review the electricity supply situation on Saturday evening and decide the prognosis for Sunday.