Power utility Eskom declared a power emergency on Friday between 16:00 and 21:00 and urged South Africans to use electricity sparingly.
"We are going to use the available resources to man the system but there is a risk of load-shedding," spokesperson Andrew Etzinger said.
"We are asking customers to use electricity very sparingly for the next five hours."
The state of emergency would remain in place until 21:00, when the situation would be reviewed.
He said the power system was severely constrained.
Eskom published load-shedding schedules for its direct customers on its website as "a precautionary measure" in the event of load-shedding.
"We are hoping to avoid load-shedding," he said.
Earlier on Friday, Eskom said it was expecting its system to remain constrained on Friday and next week.
"We are working hard to increase our reserve margin by ensuring that we have adequate emergency reserves, including water, coal and diesel supplies," he said in a statement.
He said their technicians were working to return generating units which were primarily out of service due to mechanical failures.
On Thursday afternoon, Eskom declared the power emergency, urging major industrial customers to reduce electricity consumption by at least 10%.
Etzinger called on residents to switch off geysers, air conditioners, lights and non-essential appliances.