Scorching temperatures are baking the American Southwest this Labor Day weekend. And amid the neighborhood pool parties and backyard barbeques, residents are wondering what will break first: the heat, or their power supply.

Extreme, prolonged heat waves strain electric grids because everyone cranks their power-slurping air conditioners at the same time, driving up energy demand. Meanwhile, coal, natural gas, and nuclear power plants run less efficiently and have less capacity when temperatures are high. The strain on the electric grid this weekend is so worrying that the California Independent System Operator (ISO), which oversees the state’s power system, is asking residents to not charge electric vehicles, to set thermostats to 78°F or higher, and to reduce overall energy use. The goal, it says, is to prevent energy shortages.