Troubled South African state utility Eskom on Thursday said it would extend power cuts into Friday, with energy experts predicting that outages would have to be stepped up next week as more businesses reopen after the New Year holidays.

The latest round of power cuts across South Africa, implemented sporadically since the weekend, is a reminder of the fragility of Eskom and how its unreliability has frustrated President Cyril Ramaphosa's efforts to lift economic growth.

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The power cuts have meant a baptism of fire for Eskom's new chief executive Andre de Ruyter, who started work on Monday and will oversee a government plan to split the company - which produces more than 90 percent of South Africa's power - into three units to make it more efficient.

Eskom said in a statement on Thursday that it would continue overnight cuts of up to 2,000 megawatts (MW) from the national grid until 6am local time (04:00 GMT) on Friday as it had lost additional generating capacity overnight.

The utility has been hobbled by repeated faults at its fleet of coal-fired power stations.

It often has to rely on emergency reserves such as pumped storage facilities and open-cycle gas turbines to meet power demand, but those reserves were insufficient on Thursday, when more than 14,000MW of the utility's 44,000MW nominal capacity was offline due to plant breakdowns.

Clyde Mallinson, a local energy expert, said he expected power cuts of between 4,000MW and 6,000MW next week.

Analysts say power demand could rise by around 3,000MW from January 13 as electricity-hungry companies such as factories and mines ramp up output after recent public holidays.

A three-month Eskom outlook released this week showed it would struggle to meet its reserves throughout February and March in a likely scenario with 13,700MW of breakdowns, raising the risk of further power cuts that could dent first-quarter economic output.

Power cuts implemented by Eskom last year pushed Africa's most industrialised economy to the brink of recession, with data on Thursday showing another contraction in the manufacturing sector in November.