Power utility Eskom on Wednesday called on consumers to switch off geysers, pool pumps and all non-e.
Thde incident happened in Zeerust, North West police said on Monday.
Spokeswoman Colonel Emelda Setlhako said the girls were left in the care of their grandmother on Saturday morning when their parents went to work.
“The children were playing inside the car when they fell asleep. All the doors and the windows were closed and it was extremely hot that day,” she said.
Setlhako said the mother of one of the girls arrived home and asked where the children were. The grandmother said she last saw them playing in the yard, she added.
“The mother eventually found them motionless in the car... Their skin was badly damaged due to sunburn. We suspect that they died due to extreme heat and lack of fresh air.”
Setlhako said emergency services declared the girls dead on the scene.
An inquest docket has been opened.