The power utility said that while it was able to meet Tuesday's demand without any load-shedding, challenges such as coal handling as a result of flooding at its Mpumalanga power station resulted in the loss of three generation units overnight.
"Some of these units will only be returned to service tomorrow. As a result, we had to utilise some of our emergency reserves, which include open cycle gas turbines and the pumped storage scheme, to supplement capacity."
Eskom said that its teams were working hard to bring unplanned outages back to service.
"We are monitoring the system closely and we will continue to give periodic updates on the status of the power system as things may change at short notice.
"It is important to note that the power system is vulnerable and volatile with an ageing fleet that requires higher levels of maintenance.
"Getting back to stability will take some time and we request our customers to work with us by reducing their demand while we work on improving the effectiveness of our maintenance."
African News Agency