Thirty-seven years after Sacramento voters decided to pull the plug on the Rancho Seco nuclear power plant, the land it sits on is getting a second life. The Sacramento Municipal Utility District, known as SMUD, has broken ground on a 160-megawatt, 640-megawatt-hour battery energy storage system at the former reactor site in Herald, California, layering grid-scale storage onto a property that already hosts one of the utility’s largest solar installations. Construction activity at the site was confirmed in SMUD board materials and utility communications tied to the Dry Creek Battery Energy Storage System project, which is being built as an expansion of the existing Rancho Seco Solar II development.
For the roughly 1.5 million people served by SMUD, the project is more than a symbolic gesture. A 640 MWh battery bank can store enough electricity to power roughly 64,000 average California homes for four hours, precisely the window each evening when solar generation drops off and demand peaks. That daily mismatch has forced California utilities to lean on natural gas peaker plants, and closing that gap is one of the central challenges of the state’s push toward a carbon-free grid.
From reactor to renewables
Rancho Seco’s history makes it an unusual but logical home for a giant battery. The 913-megawatt pressurized water reactor began commercial operation in 1975 but was plagued by operational problems, including a partial loss-of-coolant incident in 1978. In June 1989, SMUD ratepayers voted to close the plant permanently, making it one of the few nuclear facilities in the United States shut down by public referendum. Decommissioning took decades and cost hundreds of millions of dollars.
What remained after the reactor was dismantled was something increasingly scarce in California: a large, utility-owned parcel with high-voltage transmission lines, substation infrastructure, and industrial zoning already in place. SMUD began installing solar panels on the property in the 2000s, and the Rancho Seco Solar II project expanded that footprint significantly. The battery phase now under construction plugs directly into that existing grid connection, sidestepping two of the biggest bottlenecks that delay standalone storage projects across the state: permitting timelines and transmission queue backlogs.
The regulatory path
State environmental records provide a clear paper trail. The California Environmental Quality Act database lists the broader effort as the Rancho Seco Solar II project under State Clearinghouse Number 2017092042. That entry describes the scope as solar infrastructure combined with battery storage and interconnection components, confirming that energy storage was part of the approved plan from the start rather than an afterthought bolted onto a solar-only development.
SMUD’s formal approval is documented in a signed Notice of Determination confirming that the utility certified the Environmental Impact Report and cleared the way for construction. The battery component was filed as an addendum to the existing EIR rather than as a separate environmental review. Under California law, an addendum is appropriate when project changes do not trigger new significant environmental effects. That procedural shortcut can shave years off a timeline compared to a full new EIR, and it signals that SMUD’s environmental consultants determined the battery addition fell within the bounds of the original analysis.
Because the Rancho Seco site was already disturbed by decades of nuclear and solar operations, the incremental environmental impacts of installing battery containers and associated electrical equipment are generally more manageable than building on undeveloped land. The CEQA record indicates the project area was evaluated for habitat disturbance, visual impacts, noise, and other standard categories.
What we don’t know yet
Several important details remain outside the public record reviewed for this article. The total project cost, the identity of the battery cell supplier, and the expected commercial operation date have not appeared in the CEQA filings, which are procedural documents designed to satisfy environmental law rather than to communicate budgets or engineering specs. Those details typically surface in SMUD board meeting minutes, procurement contracts, or rate case filings.
How SMUD plans to dispatch the battery on a daily basis is also an open question. The system could serve evening peak shaving, renewable energy firming, grid contingency reserves, or some combination. California’s broader storage fleet is increasingly used for all three purposes, but no public operations plan specific to the Rancho Seco installation has been identified. Interconnection studies and grid impact analyses tied to the battery addendum may exist within the California Independent System Operator’s queue or in internal utility planning documents not yet publicly available.
How 640 MWh stacks up
At 640 megawatt-hours, the Rancho Seco battery would be large by national standards but not unprecedented in California. The Moss Landing Energy Storage Facility in Monterey County, operated by Vistra, holds roughly 3,000 MWh across multiple phases and is currently the largest battery installation in the world. The Edwards and Sanborn project in Kern County pairs solar with 1,400 MWh of storage. SMUD’s project is smaller than those utility-scale giants but significant for a municipal utility serving a single metropolitan area.
California had approximately 10,000 megawatts of battery storage capacity installed or under construction as of early 2025, according to data tracked by the California Energy Commission. The state’s grid operator, CAISO, has repeatedly cited battery storage as critical to maintaining reliability during summer heat waves, when air conditioning demand surges after sundown and solar output disappears. During a brutal heat dome in September 2022, batteries discharged more than 3,000 megawatts into the grid at peak, helping California avoid rolling blackouts. The need for that capacity has only grown since.
Retired plants as launchpads for storage
Rancho Seco is part of a broader pattern playing out across the country. Retired fossil fuel and nuclear sites are becoming prime real estate for clean energy projects because they already have the three things developers spend years chasing: land, zoning, and grid access. The former Brayton Point coal plant in Massachusetts is being redeveloped for offshore wind manufacturing and battery storage. The shuttered Becker Generating Station in Minnesota is slated for solar and storage. In each case, the logic is the same: reuse the grid connection, skip the queue, and build faster.
For SMUD customers, the practical payoff could show up as fewer hours when the utility must buy expensive power from gas-fired peaker plants on the wholesale market, costs that ultimately flow through to monthly bills. Large-scale storage also contributes to local air quality by reducing the need to fire up combustion turbines in the Sacramento Valley, an area that already struggles with some of the worst particulate pollution in the state.
For state policymakers, the project offers a working example of how California’s environmental review framework can facilitate clean energy construction when impacts are well understood and a site has already been industrialized. As the state pushes toward its mandate of 100% clean electricity by 2045, the practical challenge is no longer just building solar panels and wind turbines. It is storing what they produce and delivering it when people actually need the power. Projects like the one rising on the bones of Rancho Seco show how utilities are beginning to solve that problem, one decommissioned site at a time.
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*This article was researched with the help of AI, with human editors creating the final content.
