Nationwide, Thursday and Friday will be hotter than any time since 1950, says Hartman. "It's going to mean elevated power demand for an extended period of time for a lot of people," he says.

To meet demand, utilities are firing up special power plants used only a few days a year, delaying scheduled maintenance in order to keep all equipment on line and testing heat-sensitive switches and other equipment with high-tech devices like thermographers that can gauge temperatures to one-tenth of a degree.

"These are the days everyone wants to have their ACs on, their computers going while they watch TV," says Jon Jipping, Chief Operating Officer of ITC Holdings Corp., a transmission grid operator that owns grids in Iowa, Michigan and four other Midwest states. "These are the days we get ready for."

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