SAN ANTONIO—On a blustery February night, the Texas electricity market hit a milestone. Nearly half the power flowing onto the grid came from wind turbines, a level unimaginable a decade ago in a place better known for its long romance with fossil fuels.

The Lone Star state still embraces its oil and gas, leading a revolution in innovative “fracking” technology. Yet an equally startling energy bonanza here has gone almost unnoticed—the rise of renewables.

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