Eskom becoming South Africa’s battery

The surge in solar power adoption in South Africa is resulting in Eskom increasingly being used as a secondary electricity source and making it difficult to forecast load-shedding probabilities.

That is according to feedback from Eskom CEO Dan Marokane during the power utility’s 2025 Winter Outlook briefing.

Marokane explained that Eskom effectively becomes the country’s battery whenever rooftop solar self-generation “disappears” due to long-term overcast weather.

“Everybody then draws from the system. They don’t only draw for the immediate demand at that point in time they draw also to do recharging of the equipment,” Marokane said.

“We’ve seen in the last three weeks, frequent updates needed to be done from a forecast perspective as a result of those conditions.”

Marokane said Eskom could not bring back units under planned maintenance at a whim to fill the deficit left by the solar decline.

“When you have the combination of other reliability issues and that additional demand that’s coming through from this grouping [private solar power users], it adds to the strain on the system.”

To address the challenge, Marokane said Eskom would plan more conservatively in the future, taking into account the impact of weather changes on its ability to meet demand.

Eskom’s latest estimates put peak “behind-the-meter” private rooftop solar capacity in South Africa at roughly 6,200MW.

Therefore, if the sun is shining during the middle of the day, these installations could theoretically help the country avoid around six stages of load-shedding.

Even though the country’s peak electricity demand period is during the evenings and mornings, they still contribute significantly to the reduction in load-shedding.

National Transmission Company of South Africa (NTCSA) general manager Isabel Fick previously explained that private rooftop solar enabled Eskom to use its emergency generation more sparingly.

In previous years, Eskom had to burn more diesel to run its open-cycle gas turbines (OCGTs) and release more water from upper-level dams to power-pumped storage schemes during the day.

That would often deplete these reserves by the end of the week, plunging the country into severe load-shedding over the weekend.

The NTSCA said behind-the-meter photovoltaic solar (PV) allows pumped storage generation and OCGTs to be used as designed.

Instead of being used regularly, they are run over short periods in the morning and evening during high-demand and replenished in off-peak times.

Prolonged overcast weather can indirectly cause load-shedding

The NTCSA said there was little to no direct impact on load-shedding when PV production was reduced due to overcast conditions.

However, prolonged periods of lower PV production could result in load-shedding if peaking generation was also depleted. The NTSCA said that this had occurred frequently over the past five months.

From mid-December 2024, the northern and central parts of South Africa experienced long periods of overcast conditions, particularly in Gauteng, where much of the behind-the-meter PV is installed.

“These prolonged periods forced electricity users to return to the grid during the day, which required Eskom to supplement its generation with pumped storage and OCGTs,” the NTCSA said.

“Due to the higher usage of these generators from the beginning of a week, the fuel sources began to deplete towards the end of the week.”

“This is clearly seen by the number of weekend load-shedding events that were needed to reduce demand from the grid in order to replenish the dam and diesel levels.”

Many solar power installations also have batteries that can discharge during evening peak demand periods, reducing demand on Eskom’s grid further.

When overcast weather lasts long enough to prevent solar power users from recharging their batteries to last during the peak demand periods, the power supply shortage is amplified.

However, the NTCSA emphasised that it would be problematic to single out a single power source in South Africa as the reason why there is or is not load-shedding.

“A modern power system, such as the South African power system, derives benefits from a diverse mixture of generation technologies,” the NTCSA said.

“Some technologies have limitations under certain conditions, while others are limited for different circumstances.”

“As a System Operator, we encourage a diversity of generation technologies that can generate under different conditions and complement each other’s limitations.”