Eskom started with scheduled blackouts on Friday afternoon, the power utility said.

"Load shedding will continue until 8pm tonight," it said in a short power update.

"We appeal to customers to immediately switch off geysers, pool pumps, air conditioning and unnecessary lights and appliances to reduce the pressure on the system."

Eskom said it would release a more detailed statement later on Friday.

Eskom's load-shedding schedules were available for its direct customers on its website. Customers could also contact their call centre for additional information.

On Thursday, Eskom said the power system was severely constrained due to unforeseen technical problems at power stations.

"Should we experience further technical problems, we may need to implement load-shedding," Eskom said at the time.

Earlier this month a coal storage silo at the Majuba power station in Mpumalanga collapsed, resulting in widespread power cuts. The silo held more than 10 000 t of coal and affected coal supplies to all six units at the power station.

An investigation into the collapsed silo is underway.