By Peter Biles
BBC News, South Africa
Large parts of South Africa, including Cape Town and Johannesburg, have been experiencing power cuts, due largely to an unexpected demand for electricity.
The continuing power cuts will affect daily life across South Africa.
There has also been a partial shutdown at the Koeberg nuclear plant near Cape Town, where a turbine tripped.
The national supplier of power, Eskom, has warned that the situation will remain critical for the next week.
Eskom says there is a national alert and consumers have been warned to expect power cuts of two hours a day.
There have also been technical problems at seven power stations, with units having to be shut down.
The power cuts have caught the country by surprise. Businesses are concerned that no proper warning was given. Public Enterprises Minister Alec Erwin says there will be an investigation into the causes, but he is confident that South Africa as a whole will not be plunged into darkness.
People have been urged to switch off non-essential equipment, such as geysers, air-conditioning units and swimming pool pumps.
South Africans are not accustomed to power cuts and don't normally have back-up generators.
But last year Cape Town experienced several months of power shortages.
This led to concern about South Africa's ability to host the 2010 football World Cup.