Eskom would implement stage one blackouts from 10am on Wednesday, it said.
“There is a high demand for power and Eskom cannot meet the demand,” said spokesman Khulu Phasiwe.
He said the risk of blackouts was high for the rest of the week.
Johannesburg City Power spokesman Sol Masolo said Eskom alerted City Power just after 8am to implement the blackouts.
On Tuesday morning, Eskom said it would implement power cuts from 10am to 10pm. Around 11am, it cancelled power cuts because there was a significant drop in electricity demand.
The power utility then reverted to stage one at around 5pm and said this was due to a constrained power system. Stage one allows for up to 1000MW of the national load to be cut, stage two for up to 2000MW, and stage three for up to 4000MW. SAPA