BRUSSELS—U.S. companies are underwriting the European Union’s ambitious climate goals, fueling the bloc’s green-energy transition with power deals that cut emissions and costs.

Alphabet Inc.’s Google bought enough wind and solar energy for its EU data centers last year to power roughly a half-million European homes annually. Investments from Amazon.com Inc. are underwriting Ireland’s first wind farm to operate without subsidies, as well as renewable projects in Spain, Sweden and the U.K.

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