Great Plains Energy, the parent of regulated power utility Kansas City Power & Light, will buy rival Westar Energy for $8.6 billion, the biggest deal in the U.S. power distribution market so far this year.
Shares of Great Plains fell nearly 6 percent Tuesday, while Westar's stock gained more than 6 percent.
Falling demand for electricity in both open and regulated markets due to increased energy efficiency and a weak economy has led to several deals as U.S. utilities look to cut costs and diversify their portfolios.
The deal, which has an enterprise value of about $12.2 billion including $3.6 billion of Westar's debt, will increase Great Plains' customer base to more than 1.5 million in Kansas and Missouri and its generation capacity to nearly 13,000 megawatts.
Great Plains had a generation capacity of about 6,400 megawatts and about 850,800 customers in the two states, as of March 31.
The industry is facing increased customer expectations, stricter environmental standards and emerging cyber security threats, Great Plains Chairman and Chief Executive Terry Bassham said. "These factors, coupled with slower demand growth for electricity, are driving our costs and customer rates higher."
The deal includes a reverse breakup fee of $380 million and it is expected to close in the spring of 2017. Bassham will be the chief executive and chairman of the combined company.
Great Plains, whose last big acquisition was in 2008 when it bought Midwestern electric utility Aquila Inc, said it had secured about $8 billion of committed financing from Goldman Sachs for the Westar acquisition.
Goldman Sachs is Great Plains' financial adviser for the Westar deal and Bracewell is its legal adviser.
Guggenheim Securities is Westar's financial adviser and Baker Botts is its legal counsel.
Entering Tuesday trading, Great Plains shares were up 13.5 percent for the year, while Westar shares had gained nearly 25 percent in the same time period.
GXP (blue) and WR (green) in 2016
—CNBC's Fred Imbert contributed to this report.