Commonwealth Fusion Systems has formally applied to connect a fusion power plant to the largest wholesale electricity market in the United States, a step no other fusion company has taken. The application, filed with PJM Interconnection in May 2025, puts the Massachusetts-based company on a regulated path toward plugging its planned ARC commercial reactor into a grid that serves 65 million people across 13 states and the District of Columbia.
At the same time, CFS says its demonstration reactor, SPARC, is roughly 75 percent built at the company’s campus in Devens, Massachusetts. CEO Bob Mumgaard has used that figure in recent interviews, though CFS has not published an independently audited construction timeline. The company has also confirmed signed power purchase agreements with Google and Italian energy giant Eni, giving it committed buyers before either machine has produced a single watt of grid electricity.
Taken together, these developments represent the most aggressive public timeline any private fusion developer has put on the record. CFS is building two machines in parallel: one to prove the physics, and one to sell the power.
Why the PJM filing matters
An interconnection application is not a press release. It is a regulated filing that triggers engineering studies, cost estimates, and timeline commitments from both the applicant and the grid operator. PJM engineers will evaluate the proposed ARC plant’s technical parameters, including its expected capacity, ramp rates, and safety profile, before deciding what grid upgrades, if any, the project requires.
No other fusion company has entered PJM’s queue, which already contains hundreds of pending solar, battery, and natural gas projects. A fusion plant will be a first-of-its-kind entry, likely drawing extra scrutiny from engineers who have never modeled a tokamak’s operating characteristics on a transmission network. PJM’s interconnection backlog has stretched timelines for conventional projects to several years in some cases, so the filing marks the start of a long process rather than an imminent grid connection.
Still, the act of filing creates a paper trail with regulatory consequences. It is a concrete, externally verifiable commitment that separates CFS from fusion ventures that have stayed in the realm of announcements and fundraising rounds.
What the Google and Eni deals signal
CFS confirmed in its May 2025 announcement that it has signed offtake agreements with both Google and Eni. Offtake agreements, sometimes called power purchase agreements, commit a buyer to take delivery of electricity at a negotiated price over a set period.
Google’s interest fits a pattern. The company has been aggressively locking up carbon-free power sources to feed its expanding fleet of data centers, which consume enormous amounts of electricity around the clock. Fusion, if it works at scale, would deliver steady baseload power without the intermittency of wind and solar or the waste concerns of conventional nuclear fission.
Eni’s involvement runs deeper than a simple electricity contract. The Italian energy major has been a CFS investor since 2018 and participated in the company’s $1.8 billion Series B round in 2021. Its willingness to also sign a power purchase agreement suggests confidence that goes beyond a financial bet on a startup; it reflects a strategic decision to integrate fusion into a fossil-fuel portfolio that Eni is under pressure to decarbonize.
Neither company has disclosed the contract terms, including price per megawatt-hour, duration, or volume. Without those details, it is impossible to know whether the deals are priced competitively with other zero-carbon sources or whether the buyers are effectively subsidizing early fusion development in exchange for priority access. Both Google and Eni have signed early-stage agreements with other energy startups in the past, and not all of those projects reached completion.
SPARC: what is actually being built in Devens
SPARC is a compact tokamak, a doughnut-shaped machine that uses powerful magnets to confine plasma heated to hundreds of millions of degrees. Its purpose is to demonstrate net energy gain from fusion: producing more energy from the plasma than is required to heat and confine it. If successful, it would be the first privately built device to cross that threshold.
The core technological bet behind SPARC is a new class of high-temperature superconducting (HTS) magnets. In September 2021, CFS successfully tested a full-scale HTS magnet that achieved a field strength of roughly 20 tesla, the most powerful of its kind ever built for a fusion device. That test validated the magnet design and cleared the single biggest technical risk on the project’s critical path.
Since then, CFS has been assembling the tokamak itself at its Devens facility. The 75 percent completion figure cited by Mumgaard reflects the company’s internal tracking of construction milestones, but no independent engineering firm has publicly verified the number. What is visible from the outside is substantial: CFS has built a large-scale manufacturing and assembly campus, hired hundreds of engineers, and continued to attract private capital. The company has raised more than $2 billion to date.
The remaining work, including final assembly, systems integration, and commissioning, involves components that have never been combined at this scale. Supply chain delays, unexpected engineering challenges during integration, or performance shortfalls in custom-built subsystems could all push the schedule. CFS has not publicly committed to a specific date for SPARC’s first plasma.
The commercial plant and its permitting path
ARC is the commercial-scale reactor that CFS intends to connect to the PJM grid. It is a separate machine from SPARC, designed to generate electricity for sale rather than to prove scientific feasibility. CFS has indicated it is targeting a Conditional Use Permit for the ARC site, a local land-use approval that would allow a fusion power plant to operate in a zone where it is not permitted by default.
Local permitting for novel energy facilities can be unpredictable. Fusion plants produce no carbon emissions during operation and generate far less radioactive waste than fission reactors, but the technology is unfamiliar enough that community questions and regulatory caution could slow the process. CFS has not publicly confirmed whether the permit has been granted, denied, or remains under review as of June 2026.
The gap between the SPARC demonstration and a grid-connected ARC plant is significant. SPARC must first achieve its physics goals. Then the engineering lessons from SPARC must be translated into a larger, electricity-producing design. Construction, commissioning, and regulatory approval for ARC would follow. Each stage carries its own risks and timelines.
Where CFS stands in the fusion race
CFS is not the only private company chasing commercial fusion. Helion Energy, backed by Sam Altman, has signed a power purchase agreement with Microsoft and is building a prototype in Everett, Washington. TAE Technologies in California has raised over $1.2 billion and is pursuing a different confinement approach. Tokamak Energy in the United Kingdom is developing compact spherical tokamaks with its own HTS magnet program.
What distinguishes CFS at this moment is the combination of a validated magnet technology, a reactor under active construction, signed offtake agreements with major corporations, and a formal grid-interconnection filing. No other private fusion company has assembled all four of those elements in the public record.
That does not guarantee CFS will be the first to deliver fusion electricity. The history of fusion is littered with optimistic timelines that slipped as engineering realities set in. Government-funded projects like ITER in France have faced decades of delays and tens of billions in cost overruns. Private ventures move faster but face their own constraints: they must keep raising capital, retain specialized talent, and solve problems that have never been solved before.
What to watch next
The most telling milestones in the months ahead will not come from press releases. They will come from regulatory filings, permitting decisions, and, eventually, plasma data.
Specifically, readers and analysts should track whether PJM advances the CFS interconnection application through its study process, whether the Conditional Use Permit for the ARC site is granted, and whether CFS provides updated, verifiable construction data for SPARC. Each of these creates a public record that can be checked independently.
The balance of evidence as of June 2026 supports a cautiously optimistic reading: CFS is further along the path to a commercial fusion plant than any other private company that has documented its progress through regulated filings and confirmed corporate agreements. But substantial technical, regulatory, and economic uncertainties remain between where the company stands today and the moment fusion power actually flows into the grid. The paper trail, not the promises, will tell the story.
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*This article was researched with the help of AI, with human editors creating the final content.
