The deal consists of $3 billion in cash and Energy Harbor shareholders getting a 15% stake in a newly formed subsidiary holding company called Vistra Vision.
Shares of Vistra gained 5.1% to $22.5 in premarket trade.
The deal, which underscores a push by U.S. utilities to invest in shifting to clean energy, would help Vistra add about 4,000 megawatts of nuclear capacity and about a million retail customers. The company also announced additional share buybacks worth $1 billion.
The nuclear and retail businesses of both companies will be combined, with Vistra Zero renewables and storage projects under Vistra Vision.
The unit will assume about $430 million of net debt from Energy Harbor following the deal's expected close in the second half of 2023.
Energy Harbor operates the second-largest non-regulated nuclear fleet in the United States, supplying roughly 33 terawatt hours of carbon-free power generation annually. The company was earlier known as FirstEnergy Solutions and emerged as Energy Harbor following a restructuring in 2020.