"The power system is severely constrained ... due to unforeseen technical problems at power stations."
"Should we experience further technical problems, we may need to implement load shedding."
The power utility called on all consumers to save energy consumption.
Eskom also issued an alert on Tuesday over an increased demand for electricity, given cold and wet weather.
South Africans were caught off-guard by a sudden dose of load shedding earlier this month after a coal silo collapsed at Eskom's Majuba power station in Mpumalanga.
An investigation by the power utility into the collapse of a coal storage silo could take up to six months, Sapa reported.
Eskom calls on consumers to switch off geysers, air conditioners, pool pumps, non-essential appliances from 5pm - 9pm to avoid load shedding
— Eskom Media Desk (@Eskom_MediaDesk) November 20, 2014
Etzinger: Technical problems arose at “power stations which have not received the maintenance it needed” #Eskom.
— Jacaranda Newsteam (@JacaNews) November 20, 2014
Eskom has alerted us that load shedding may resume between 5pm and 10pm tonight. Help prevent this by switching off unnecessary appliances.
— Western Cape Gov (@WesternCapeGov) November 20, 2014
#Eskom says high probability of loadshedding tonight "due to unforseen problems at power stations". It never ends! :-(
— Chris Yelland (@chrisyelland) November 20, 2014