This is how Eskom makes the call on which lights to switch off and when

Sitting almost serenely on Power Street in Germiston, on Gauteng's east rand, is the "brain" of Eskom's operations, where a group of engineers and technicians keep a close eye on an overhead screen.

This is Eskom's National Control Centre, where the call is made on which lights to switch off and when.

Each power station supplying the grid communicates its energy output capabilities to the centre. The data is used to calculate how much energy is needed by each area and how much the power utility can supply.

Eskom can shift power anywhere within the power grid. For example, the new solar-power plant in Kathu in the Northern Cape can send power to Gauteng.

Load-shedding is enforced as a last resort when the combined output of the power stations is not enough to supply the grid.

If the grid were to be left to trip, the result would be a nationwide blackout that, according to Eskom, could take days to restore. The short-term fix is load-shedding.

"When power is insufficient, Eskom can either increase supply or reduce demand to bring the system back into balance," Eskom said in response to Sunday Times questions.

As the gap between supply and demand widens, the team starts gearing up to prevent the system from becoming unstable.

First, the utility asks larger customers, like mines, to cut their load voluntarily.

"However, if several power station units trip suddenly and unexpectedly, we may have to skip those steps and go straight to load-shedding to prevent the system from becoming unstable," said Eskom.

Eskom's grid is fed by various suppliers, with a potential 55,330MW from coal-fired, wind, solar, hydroelectric and nuclear power, pumped storage schemes and gas turbines. Electricity is moved from generator to metros and municipalities and ultimately to homes along 359,337km of power lines.