Company Profile / Energy & Utilities
The Company Betting America's Grid on a Reactor It Once Switched Off
It runs the largest nuclear fleet in the country, sells power to three-quarters of the Fortune 100, and just agreed to bring a piece of Three Mile Island back online for Microsoft. Inside the utility quietly rewiring how America pays for clean power.
In 2019, a nuclear unit on an island in the Susquehanna River went quiet. It was not a safety problem. It was arithmetic - cheap natural gas and cheaper renewables had made Three Mile Island Unit 1 uneconomic to run. Five years later, the same reactor is being brought back to life, and a software company in Redmond has already agreed to buy two decades of its output. The company flipping that switch is Constellation Energy, and the reversal says almost everything about where American power is heading.
Constellation is not a startup, and it does not behave like one. It is the largest producer of carbon-free electricity in the United States, headquartered in Baltimore and traded on the Nasdaq under the ticker CEG. Its economic core is the biggest fleet of nuclear reactors in the country, wrapped in hydro, wind, solar and gas. On an ordinary day that fleet runs nearly 90% carbon-free and generates enough electricity to power the equivalent of about 15 million homes - roughly a tenth of all the clean energy produced in America.
01 / The SpinoffThe boring half that turned out to be the whole game
Constellation as it exists today is young. In February 2022, the utility conglomerate Exelon split itself in two. On one side went the regulated wires-and-poles businesses - the local utilities that deliver electricity and send the bills. On the other side went the power plants, the trading desk and the competitive retail business. That second company was Constellation Energy Corporation. At the time it read as the less glamorous half: no guaranteed regulated returns, just a fleet of aging reactors exposed to volatile wholesale markets.
The market's judgment did not stay there for long. As data centers, electrification and reshored manufacturing began pushing electricity demand up for the first time in years, the value of always-on, carbon-free generation climbed with it. The plants nobody wanted became the asset everybody needed.
02 / What It SellsNot just electrons - a way to keep a promise
Constellation makes money in two broad ways. It generates power from its own plants and sells it into wholesale markets, and it supplies electricity, natural gas and energy services directly to end customers in states where retail energy is deregulated. Those customers span homes, small businesses, factories, city governments and community aggregations across the continental US - including, by the company's account, roughly three-quarters of the Fortune 100.
The more interesting layer sits on top of the commodity. Large companies have made public climate commitments, and those commitments need a supplier who can help meet them. Constellation sells energy efficiency, on- and off-site clean energy and storage, greenhouse-gas monitoring and reporting, and structured renewable products such as its Constellation Offsite Renewables offerings. Its headline offering is 24/7 carbon-free matching - matching a customer's electricity use to clean generation hour by hour, rather than buying an annual pile of renewable-energy certificates and calling it even. It is a harder promise to keep, which is exactly why it is worth selling.
The customer base runs the full range. On one end are households picking a competitive supplier for their monthly bill; on the other are hyperscale operators signing contracts long enough to justify restarting a reactor. In between sit municipalities, cooperatives, hospitals, universities and industrial plants that treat electricity as both a cost line and a climate obligation. That breadth is a hedge: retail relationships smooth out the swings of wholesale power prices, and corporate demand for clean supply gives the fleet a premium buyer for the exact product it already makes.
03 / The Microsoft DealBuying twenty years of a reactor that was closed
In September 2024, Constellation announced the move that put it on front pages. It would restart Three Mile Island Unit 1 - the reactor idled in 2019, not the one involved in the 1979 accident - and rename it the Crane Clean Energy Center, after Chris Crane, the late chief executive of Exelon. The economics that made a restart possible came from a single customer: Microsoft signed a 20-year power purchase agreement to buy the plant's output and match its data-center electricity with carbon-free power. The unit is targeted to return to service around 2028.
The scale of the arrangement is unusual. Constellation has said the project is expected to create roughly 3,400 jobs and add on the order of $16 billion to Pennsylvania's economy. More than the numbers, the deal reframed a decades-old story: the same site synonymous with nuclear anxiety became a symbol of nuclear's second act, this time underwritten by the appetite of artificial intelligence for reliable, clean electricity.
04 / The Calpine BetBecoming the biggest generator in the country
If the Microsoft deal was the symbol, the Calpine acquisition was the scale. In January 2025 Constellation agreed to buy Calpine - the largest US producer of power from low-emission natural gas and the operator of the country's biggest geothermal fleet - at an equity purchase price of $16.4 billion. Federal regulators at FERC approved the merger in July 2025, subject to conditions meant to limit the combined company's market power in the PJM grid, and the transaction closed in January 2026.
The result is a generation platform of roughly 60 gigawatts spanning nuclear, gas, geothermal, hydro, wind, solar and battery storage - making Constellation the largest power generator in the United States. It is a portfolio built for a grid that needs both carbon-free baseload and flexible capacity to fill the gaps.
05 / The Market PositionWhere it fits, and who it is up against
Constellation sits at the intersection of two industries that used to be treated separately: power generation and corporate sustainability. It competes on the generation side with independent power producers and large utilities - names like Vistra, NRG Energy, PSEG, Dominion and Duke - for both the electrons and the retail relationships. What sets it apart is the composition of its fleet. Most competitors chasing clean-energy demand are building it. Constellation already owns the single largest source of carbon-free power in the country and is expanding it rather than starting from scratch.
The company's economics are also shaped by policy. Federal clean-energy incentives, including the nuclear production tax credit, help underwrite the value of a reactor fleet that is carbon-free but expensive to operate. That is part of why a plant like Crane can pencil out today when it could not in 2019 - the value of clean, firm power, and the willingness of buyers to pay for it, has changed.
06 / How It Got HereA short timeline of a fast four years
- 1979 Three Mile Island accident. Unit 2 makes the site a household name in the nuclear-safety debate.
- 2019 TMI Unit 1 shuts down, unable to compete with cheap gas and renewables.
- 2022 Constellation goes public, spun out of Exelon and listed on the Nasdaq as CEG.
- 2024 Microsoft PPA and TMI restart announced; the plant is renamed the Crane Clean Energy Center.
- 2025 Calpine deal signed for $16.4B; FERC approves the merger in July.
- 2026 Calpine acquisition closes, making Constellation the largest US power generator.
07 / The TakeawayThe unglamorous layer under the shiny one
Most of the attention in the AI era goes to chips and models. The quieter constraint is power - not the abstract kind, but megawatts that arrive on schedule and do not emit carbon. Constellation has spent the last four years positioning itself on the selling side of that constraint: a Baltimore company with about 16,000 employees, roughly $25.5 billion in annual revenue, and the largest fleet of carbon-free generation in the country. Whether the bet on nuclear's revival holds will be decided over decades, on 20-year contracts and reactors that outlast news cycles. For now, the switch on the Susquehanna is being turned back on.