Eskom will begin operating Unit 3 at the Kusile coal-fired power station this week, the state-owned utility said on Wednesday, marking the halfway point in a R118.5bn project which began in 2008.

"This brings to three the number of generation units that have achieved commercial status at the project, generating a maximum 2,400MW to support the SA power grid," Eskom said in a statement.

Eskom provides more than 90% of the electricity needed by SA, but has had to implement nationwide blackouts due to repeated faults at its ailing coal-fired power stations, severely constraining economic growth.

In February and March it cut power regularly in order to perform maintenance at its plants.

This month it faced uncertainty on about 6,000MW of its nominal capacity of more than 44,000MW and expects electricity supply to remain volatile until maintenance is completed by September.