The artificial-intelligence boom’s promise of runaway electricity demand has jolted shares of U.S. power companies to all-time highs. But those generators and utilities are now learning that the hype comes with an edge: Investors won’t wait forever for results.

Companies that recently hit record valuations are returning to earth as investors realize the massive data-center deals they’d banked on are actually smaller, or slower, than expected.

Constellation Energy Corp. saw shares tumble 11% from an October high after a third-quarter earnings call that yielded no details on new power generation. The headline of one Jefferies analyst note read: “No Data Center Deals.” Similarly, Vistra Corp. has fallen 16% since mid-October as analysts noted a slower pace of data center announcements than

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