Calpine Corporation is the largest generator of electricity from natural gas and geothermal resources in the United States, with operations in competitive power markets.
 A Fortune 500 company based in Houston, Texas, the company is owned by an affiliate of Energy Capital Partners and a consortium of other investors, including Access Industries Inc. and the Canada Pension Plan Investment Board.
 Through wholesale power operations and its retail businesses, Calpine serves customers in 24 states, Canada, and Mexico.
 Its fleet of 76 power plants in operation or under construction represents nearly 26,035 megawatts of generation capacity. In 2019, it reported generating 100.8 megawatt hours of electricity, 5% more than in 2018. The name "Calpine" is derived from the company's original California location and alpine, a reference to the Zürich home base of Electrowatt. Calpine is the largest generator of electricity from natural gas and geothermal resources in the United States.
 Its grouping of 19 power plants located in a geothermal field outside Sonoma, CA often referred to as "The Geysers"  is the largest producer of renewable geothermal power in North America, producing 725 megawatts of electricity, enough to power 725,000 homes or a city the size of San Francisco. Despite the name of the steam field no natural geysers exist near The Geysers - Clear Lake area.
 In 1988, the first QF cogeneration plant was commissioned and power production began. In 1992, the company's assets reached US$21 billion. In 1994, the company reached capacity output of 141 MegaWatts. In 1996, the company's initial public offering (IPO) was the largest for an independent energy company.
 In 1998, Calpine purchased 45 gas turbine power plants. The following year it purchased 18 gas turbine power plants and acquired PG&E's plants at The Geysers, making Calpine the world's largest geothermal provider.
 In 2001, the California electric energy crisis occurred. In 2004, the investment bank Lehman Brothers begins shorting Calpine, with researcher Christine Daley lacking confidence in Calpine. This information spreads to clients of Lehman. By the time Calpine goes bankrupt in 2005, Lehman will profit roughly $100,000,000 from the short. In November 2005, CEO Peter Cartwright and CFO Bob Kelly step down and left the company. On December 20, Calpine filed bankruptcy with US$22 billion in debt. Calpine's expansion plan was unsupportable in the economic environment formed by the 2000-2001 California energy crisis and the collapse of Enron.  Stock price dropped to less than US$0.30 per share, and was delisted from NYSE. 
 On January 31, 2008, Calpine returned from bankruptcy. The company's previous stock was exchanged for warrants and new Calpine stock began trading on the NYSE under the ticker symbol "CPN."Later that year, a new executive leadership team, headed by president and CEO Jack Fusco, joined the company. 
 In 2009, the company moved its corporate headquarters from San Jose, California to Houston, Texas.[citation needed] In 2010, Calpine acquired Conectiv Energy (generation) from Pepco Holdings. In 2010, Calpine dedicated the Geothermal Visitor Center and celebrated 50 years of geothermal power production at The Geysers in Northern California.[citation needed] In 2010, Thad Hill was named chief operating officer.[citation needed]
 In 2012, Calpine's power plant fleet generates a record 116 million MWh of electricity. In 2013, Calpine rang the opening bell at the New York Stock Exchange on January 18.  In 2014, Jack Fusco became Calpine's executive chairman of the board and Thad Hill became the company's chief executive officer. In 2014, Calpine sold six power plants in its Southeast region. In 2015, Calpine acquired the electric provider Champion Energy, expanding channels in its core Texas and Northeastern U.S. markets. In 2016, Calpine acquired Granite Ridge Energy Center in Londonderry, New Hampshire, Frank Cassidy became chairman of the board, and it acquired Noble Group Ltd's North American energy business. In 2018, Calpine was acquired by an affiliate of Energy Capital Partners and a consortium of other investors, including Access Industries Inc. and Canada Pension Plan Investment Board, on March 8 a new board was named. Shares of Calpine's common stock stopped trading prior to the March 9, 2018, opening of the New York Stock Exchange.[citation needed]
 In February 2023, Calpine released plans to begin development for a 425 MW natural gas-fired plant next to the Freestone Energy Center in Freestone County, Texas. In July 2023, Calpine announced a $25 million carbon capture technology project. The technology could capture 95% of a plant’s carbon emissions, which would reduce greenhouse emissions. The technology was created by ION Clean Energy of Colorado.