Spain's energy reforms, which aim to cut a massive tariff shortfall in the regulated power system, have wiped 1.4 billion euros from Iberdrola's earnings since 2011.
Angry over the painful reforms, Galan vowed to cut domestic investments drastically and boost exposure abroad, especially in the United States and Mexico.
Iberdrola said UIL shareholders will receive one share in the new company for each share they own and an additional cash payment of $10.50 per share.
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The proposed deal implies a total value of $52.75 per share, which includes the cash component, representing a 25 percent premium to UIL's closing price on Feb. 25, the companies said.
Iberdrola will combine its U.S. unit with UIL and list the new company on a U.S. exchange. The companies did not specify the exchange on which it would be listed.
The new company will serve 3.1 million electric and gas customers across New York, Connecticut, Maine and Massachusetts and would invest $6.9 billion in electric and gas infrastructure over the next five years, the companies said.
UIL Chief Executive James Torgerson will be the new company's CEO. Iberdrola and UIL will continue to have offices in New Haven, Connecticut, Massachusetts, Maine and New York.
Energy East, now known as Iberdrola USA, delisted from the New York Stock Exchange in 2008 and became a wholly-owned subsidiary of Iberdrola SA.
The deal, which has been unanimously approved by both companies' boards, is expected to close by the end of 2015. Morgan Stanley served as financial adviser to UIL, while Sullivan & Cromwell and Wiggin and Dana served as legal advisers.