Wisconsin Energy Corp., owner of that state’s largest utility, agreed to buy Integrys Energy Group Inc. for $5.7 billion in cash and stock to almost double its customer base and increase revenue.

Wisconsin Energy, which is based in Milwaukee, will pay 1.128 a share and $18.58 in cash for each share of Integrys, representing a 17 percent premium to Integrys’s June 20 closing price, the companies said today in a statement. The deal is expected to close in the summer of 2015 subject to state and federal regulatory approvals.

Its the largest U.S. utility purchase announced this year behind Chicago-based Exelon Corp.’s $6.8 billion cash offer for Pepco Holdings Inc., owner of Washington’s electric utility. Louisiana utility owner Cleco Corp. said today it has been approached by third parties and that Goldman Sachs & Co. and Tudor, Pickering, Holt & Co. have been retained as financial advisers.

“We’ve had a lot of utility mergers for slightly different reasons,” Kit Konolige, a New York-based analyst for BGC Partners LP, said today in an interview. “There are significant operating and financial efficiencies that you can get from combining somewhat smaller companies into a larger one.” Konolige, who owns no shares, rates Wisconsin Energy at hold.

The combined company, WEC Energy Group, will have 4.3 million gas and electric customers inWisconsin, Illinois, Michigan and Minnesota, including those of Peoples Gas, a Chicago utility. The value of the transaction will be $9.1 billion including assumed debt.

Chicago-based Integrys shareholders will own about 28 percent of WEC Energy.

“It’s a great deal for Integrys shareholders,” said Charles Fishman, a St. Louis-based analyst for Morningstar Inc., who valued Integrys at $54 a share. “It’s a real healthy premium and they’re getting shares in a very solid company.”

Integrys has missed analysts’ per-share profit expectations in four of the past eight quarters, according to data compiled by Bloomberg. Wisconsin Energy’s earnings per share have exceeded the estimates for 18 consecutive quarters.

Integrys also said today it is in the “late stages” of divesting its unregulated retail-marketing unit, Integrys Energy Services.

Wisconsin Energy Chairman and Chief Executive Officer Gale Klappa will be chairman and CEO of the combined company. Integrys chairman and CEO Charlie Schrock will retire upon closing of the deal.

“We have been consistent in our commitment to undertake acquisitions or combinations only if we believe they will be accretive to earnings per share in the first calendar year after closing, be largely credit neutral and produce a growth rate at least equal to Wisconsin Energy’s stand-alone growth rate,” Klappa said in the statement.

The purchase will increase profit in the first full year after closing, the companies said today in slides posted for an investor call. Earnings per share will rise 5 percent to 7 percent annually over the long-term.

Barclays Plc and Skadden, Arps, Slate Meagher and Flom LLP advised Wisconsin Energy. Lazard Ltd., Cravath, Swaine & Moore LLP and Foley & Lardner LLP advised Integrys.

The announcement was made before regular trading began today in New York. Integrys rose 16 percent to $70.90 at 7:28 a.m. in New York. Wisconsin Energy rose 0.02 percent to $46.90

Exelon, the largest operator of U.S. nuclear-power plants, agreed to buy Pepco for $6.8 billion in cash in April. UIL Holdings Corp. has bid $1.86 billion for city-owned Philadelphia Gas Works. The Laclede Group Inc. is buying Alabama Gas Corp. from Energen Corp. for $1.34 billion.