South Africans need to use electricity more sparingly this week because the system is "tight", Eskom said on Thursday.
"The system is tight and customers are urged to switch off all non-essential appliances, particularly over the evening peak," it said in a system status bulletin.
"We urge all South Africans to partner with us to save 10 percent of their electricity usage, particularly during the... peak hours from 5pm to 9pm."
It said a unit at the Koeberg nuclear power station in Cape Town was being returned to service after automatically shutting down on Wednesday because of an electrical fault.
"The reactor is in a safe and stable condition and there has been no interruption to power supply as a result of the shutdown."
Total demand on Thursday evening was forecast to reach 35,043 megawatts (MW).
Eskom had 35,514MW of capacity available, including from open cycle gas turbines.
Current planned maintenance accounted for 2306MW. Unplanned outages affected 4717MW.
On Wednesday, peak demand of 34,094MW was met, with an available capacity of 35,771MW.
On Tuesday, peak demand of 34,212MW was met, with an available capacity of 37,128MW.
Peak demand for the rest of the week was forecast to reach 33,634MW on Friday, 32,082MW on Saturday, 32,485MW on Sunday, 35,187MW on Monday, 35,087MW on Tuesday and 35,305MW on Wednesday.