South Africa's state-run electricity company Eskom has apologised for the latest round of power cuts that began on Wednesday.
The blackouts will affect different parts of the country throughout the day.
In a statement, Eskom said its ability to generate power had been hit by maintenance problems.
“The severe supply constraint being experienced has come about due to high levels of unplanned breakdowns... [for example] five generating units are unavailable due to boiler tube leaks,” it said.
"We unreservedly apologise to South African for the negative impact this may have on them," the company added.
It also advised people to use power "sparingly" through the day.
This is not the first time the utility has resorted to load shedding. Similar measures were introduced earlier this year and last year, as well as in 2008 and 2015.
The problems have been caused by a combination of increasing demand, rising costs, falling revenues, crumbling infrastructure, and decades of corruption and mismanagement.
More than half of Eskom's debt is guaranteed by the government, accounting for 15% of the national debt. International banks have described Eskom as the single biggest threat to South Africa's economy.
Image caption: Most of Eskom's power plants run on coal, an abundant resource in South Africa