Major industrial customers were urged to reduce electricity consumption by at least 10 percent on Thursday as Eskom declared a power emergency.
"The power system is severely constrained today due to the loss of additional generating units from our power station fleet, reduced imports, and the extensive use of emergency reserves," Eskom said in a statement.
This forced Eskom to declare an emergency, in accordance with its regulatory protocols, to protect the national power grid.
Eskom published load shedding schedules for its direct customers on its website, but said these were only "a precautionary measure".
"While we will make every effort to avoid load shedding, we are hopeful that by applying these measures, coupled with support from all electricity consumers, we will achieve the required load reduction necessary to protect the national grid."
The state of emergency would remain in place until 9pm, when the situation would be reviewed.
"We have alerted our key industrial customers and have required them to reduce their load by a minimum of 10 percent as from 2pm [on Thursday]."
Shopping centres and retail outlets were asked to switch off geysers and adjust air conditioning to 23 degrees Celsius to help reduce demand.
"As workers leave office buildings this evening, we ask that they please switch off lights with the exception of the security lighting, and avoid leaving on air-conditioning overnight."
Residential customers were asked to switch off geysers, air conditioners, lights and non-essential appliances between 5pm and 9pm.