State power company Eskom said it would implement rolling blackouts for a third straight day on Tuesday, taking 3 000 MW off the load at any given time to ease pressure on the national grid.
It said generating units that had broken down unexpectedly had returned to service, but emergency reserves were still very low.
"Eskom will implement Stage 3 rotational load shedding from 08h00 today and is likely to continue until 23h00," the utility said.
"Stage 3 calls for 3 000 MW to be rotationally load shed nationally at a given period. Load shedding is conducted rotationally as a measure of last resort to protect the power system from a total collapse or blackout."
Reports said parts of the central business district in South Africa's capital Pretoria had already been in the dark since around 6 am on Tuesday, causing heavy traffic jams.