Steenbras dam holds over 65 billion litres of water for the Western Cape. By releasing stored water to the lower dam, Steenbras acts as a 180 MW power station during severe loadshedding. When power is more available, electricity is then pulled from the grid to pump water back up to the upper dam – where it is held ready for severe loadshedding again. In this way, the dam keeps Cape Town on lower stages of loadshedding than experienced nationally

Source: one, two, three