WASHINGTON, July 4 -- On the 250th birthday of the United States, Philadelphia cancelled its Independence Day parade. Boston delayed entry to its riverside fireworks celebration by four hours. And the operator of the country's largest power grid, which serves 67 million people, issued an electricity emergency it warned could last through midnight Saturday.

The cause was not logistics or labor. It was heat. Temperatures across the eastern United States crested at heat indices of 113 degrees Fahrenheit in Washington, 112 degrees in Philadelphia, and 110 degrees in New York City on Friday, part of a wave that placed 160 million Americans in 30 states under extreme-heat alerts. PJM Interconnection, the grid operator managing power across a 1...