Eskom has declared an electricity supply emergency, and is urging customers to severely curtail usage from Tuesday onwards.

"The power system is severely constrained today due to the loss of additional generating units from our power station fleet and the extensive use of emergency reserves," Eskom said in a statement on Tuesday.

"This has necessitated the need for Eskom to declare an emergency in terms of the approved regulatory protocols."

Eskom was now following protocols under its emergency procedures to secure the power system.

"We have alerted our key industrial customers and have required them to reduce their load by a minimum of 10 percent," the parastatal said.

"While we will make every effort to avoid load shedding, we are hopeful that by applying these measures we will achieve the required load reduction necessary to protect the national grid."

Eskom called on all customers to switch off their geysers, pool pumps, air conditioning and all non-essential appliances between 5pm and 9pm.

"As workers leave office buildings this [Tuesday] evening, we ask that they please switch off lights, with the exception of the security lighting," Eskom said.

"As a precautionary measure, we will be publishing load shedding schedules for Eskom direct customers on our website from 5pm today."

The web address is loadshedding.eskom.co.za/.

Customers could also contact the call centre at 086-003-7566 for additional information.

"Load shedding schedules will only be utilised for the duration of the constraint and Eskom will provide regular updates on the status of the power system," it said.

"Where possible, we recommend that you switch off air conditioning unless necessary, in which case it should be set at 23C."

The company urged the public to look out for power alerts and bulletins on television to find out the real-time status of the electricity system.