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 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 they exp

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