Joburg businesses could cut grid use by 97% with rooftop solar, says UJ study

A mid-sized Johannesburg business could cut the electricity it buys from the grid by 97.7% by putting solar panels and batteries on its roof, a University of Johannesburg (UJ) study has found.

Researchers modelled a commercial building in Melville that uses about 840 kWh of electricity a day, with a peak load of about 178kW, more than five times its average. In the model, its grid consumption falls from 306,618kWh a year to 7,011kWh, and its batteries can keep it running for about 18 hours during an outage.

The design pairs a 337kW solar array with 901kWh of battery storage and a bi-directional inverter, which controls how power moves between the panels, the batteries and the grid.

Associate professor Bonginkosi Thango, of the UJ’s department of electrical engineering technology, said the aim was not to cut the cord completely.

“Complete off-grid independence is expensive,” Thango said. “This is not an ‘off-grid’ model. It is a resilience model. The facility keeps its grid connection while using PV and storage to make the grid a smaller part of the energy mix.”

The batteries matter because the building’s demand is strongest between 6pm and 9pm.

“That profile is consistent with a busy commercial building using lighting, cooling, refrigeration and electronic equipment,” said Thango.

According to Thango, solar panels on their own could not supply the building at night, or through a grid outage, unless the system had storage.

Eskom has commissioned utility-scale battery projects, such as the Hex battery energy installation in the Western Cape, and businesses are increasingly buying their own.

But Thango cautioned that businesses should not expect the same results.

“A 97.7% cut is technically achievable for a well-sized site with sufficient solar generation, storage and effective energy management, but it should not be treated as a standard percentage for every Johannesburg business,” he said.

He said owners should treat solar “as an operating-cost and business-continuity decision, not only as an environmental investment”.

Separate UJ research on 15 Gauteng enterprises shows how much the picture varies. A hair salon using about 11.7kWh a day would take 6.46 years to recover the cost of a grid-linked solar system, while a hotel kitchen using about 258kWh a day would get its money back in 4.3 years.

Thango estimated the 337kW array would cost about R4.04m to R4.55m at current 2025/26 South African market benchmarks, and the battery system about R3.60m to R6.31m. That puts the full installation at between R7.65m and R10.86m.