As the United States raced over the past five years toward becoming a global petroleum powerhouse, the world’s biggest oil exporter Saudi Arabia quietly seized a market milestone from America: the largest source of peak summer demand. From June through August, when temperatures in Riyadh routinely rise above 100 degrees Fahrenheit (38 degrees Celsius), Saudi Arabia diverts as much as a tenth of its crude output to fuel power plants that run full tilt to meet surging demand from air conditioners.