Pinnacle West is a holding company whose principal subsidiary is Arizona Public Service, a vertically integrated electric utility that serves 1.4 million customers across a 35,000-square-mile territory in central Arizona, including Phoenix. APS owns or leases more than 6 gigawatts of power generation capacity, including a 29% ownership stake in the largest nuclear plant in the US, Palo Verde. About half of the electricity that APS supplies to customers comes from clean energy sources, including nuclear.