This is a delay of at least six months caused by labour unrest and "underperformance" by contractors.
"The target date of December 2013 is unlikely to be achieved. A more realistic target date for first power from Unit 6 is the second half of 2014," Eskom chief executive Brian Dames told a news conference.
Eskom, which provides 95 percent of the power to Africa's biggest economy, has been walking a tightrope for five years as it tries to bring long-overdue power plants online after the grid came close to collapse in 2008.
The power crisis forced factories, mines and smelters to shut down for days, costing the economy billions of dollars in lost output.
Already this year, the grid has sometimes operated with a buffer of less than 1 percent of its capacity.
Eskom's hopes had rested on the completion of the massive Medupi power plant but construction of the coal-fired power station has been beset by delays due to labour disputes.
Construction of a second coal-fired power station, Kusile, was also disrupted by strikes. Kusile is due to come on line next year.
Dames said Eskom was still trying to work out what impact the labour disruption has had on Kusile's start date.