JOHANNESBURG, South Africa — As schools reopen here in the coming days after the Christmas break, South Africans are braced for a surge in the power cuts that have plagued the country since early December, and the rolling blackouts could spell trouble for the ruling African National Congress at the ballot box. During the week, schools, factories and offices will move to full capacity, drawing on an already-fragile national grid. And in the heat of the Southern Hemisphere summer, air conditioners will be turned on full blast. A state-owned company, ESKOM, has the monopoly to produce more than 95 percent...