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| Fig. 1: Diablo Canyon Power Plant in San Luis Obispo County, California. (Image Source: Wikimedia Commons). |
The Diablo Canyon Power Plant (see Fig. 1) is California's only operating commercial nuclear power plant and one of the state's most important single electricity-producing facilities; California has not built a new nuclear plant in decades because of its waste-disposal moratorium. Since it is a large, steady baseload generator, its annual electricity output is a useful summary measure of both its technical scale and its role in the California grid. Unlike solar and wind resources, whose annual output varies strongly with weather conditions, Diablo Canyon is designed to operate at high capacity for long periods between refueling outages. For that reason, the plant's yearly generation can be described with a fairly stable round number that is cited consistently across official state and federal energy sources.
Two independent California Energy Commission reports converge on the same figure. The Diablo Canyon Power Plant Operations Assessment Report states that the plant produces about 18,000 gigawatt-hours of electricity annually, and the 2024 Senate Bill 846 Diablo Canyon Power Plant Extension Cost Comparison uses that same 18,000 GWh/year benchmark as its baseline when evaluating the economics of extended operation against replacement portfolios. [1,2] The consistency across two distinct CEC analyses, one operational and one financial, gives the number credibility beyond what either source alone would provide.
That annual total is worth translating into more intuitive terms. Averaged across every hour of the year, 18,000 GWh corresponds to a continuous electrical output of roughly 2.05 GW, a sustained power level that few single facilities in the Western United States can match. The 2025 CEC assessment adds further texture: Diablo Canyon contributes approximately 2,280 MW to California's peak and net-peak capacity needs and supplies about 8.2 percent of the state's in-state electricity generation. [1] Taken together, these figures paint a picture of structural importance: Diablo Canyon is, by any reasonable measure, one of the largest and most consequential single electricity-producing assets in the state.
First, Diablo Canyon matters because California no longer has other operating nuclear plants of comparable size. The 2025 CEC assessment states that Diablo Canyon has been California's last operating nuclear power plant since 2012, and its appendix on nuclear energy in California explains that Humboldt Bay, Rancho Seco, and San Onofre are in various stages of decommissioning or already decommissioned. [1] The same appendix notes that Rancho Seco operated until 1989 and that San Onofre Units 2 and 3 were permanently retired in 2013. [1] So if Diablo Canyon closes, California does not replace it with "other nuclear"; it replaces it with some combination of renewables, storage, imports, and gas-fired generation.
Second, the number matters because California has made it very difficult to build another plant like Diablo Canyon. The same 2025 CEC report explains that California placed a moratorium in 1976 on the construction and licensing of new nuclear reactors until the federal government identifies and approves demonstrated technology for fuel reprocessing and permanent disposal of high-level nuclear waste, and it states that these conditions have in effect halted the construction of new nuclear power plants in California. [1] A 2024 California Senate floor analysis likewise describes California law as imposing a de facto moratorium on new nuclear fission reactors until the federal government approves a means for disposal of high-level nuclear waste. [3] That makes Diablo Canyon unusual since California's own legal framework has made successors hard to build.
Third, Diablo Canyon operates in a grid whose main challenge is increasingly not total annual energy, but timing. In its 2021 Report to the Governor on Priority SB 100 Actions to Accelerate the Transition to Carbon-Free Energy, the CEC states that the near-term reliability challenge is operating through the "net demand peak" period and explains that, with today's resource mix, solar generation declines in the late afternoon faster than demand decreases. [4] The report further notes that this steep ramp occurs because loads served by solar during the day need replacement electricity as solar generation drops toward zero. [4] In that setting, Diablo Canyon's annual generation is important not because it wins the noon-hour production contest against solar, but because it is firm generation that does not disappear at sunset.
Finally, Diablo Canyon is politically notable precisely because it sits at the intersection of two California energy preferences that do not fit neatly together. California strongly favors renewable generation, but it also still needs dependable low-carbon electricity. The same 2025 CEC assessment explains that SB 846 was passed because Diablo Canyon provides reliable power and because of challenges in developing enough new clean energy projects in time to support grid reliability. [1] More broadly, the technical literature also suggests that firm low-carbon resources can reduce the cost and difficulty of deep decarbonization, and a 2021 MIT assessment argues that Diablo Canyon may also have value beyond electricity production alone. [5,6] The 18,000 GWh/year figure is therefore not only a production statistic; it is a measure of how much firm carbon-free electricity California still gets from a technology it has largely chosen not to reproduce.
Using current California Energy Commission reporting, supplemented by a second CEC planning study, Diablo Canyon Power Plant produces about 18,000 GWh of electricity per year. [1,2] This is represents roughly 8-9 percent of California's electricity generation, depending on whether the comparison is made to in-state generation alone or to total electricity supply. Those numbers alone make it one of California's most important power plants.
At the same time, the more interesting story is what the numbers imply. Diablo Canyon is California's last operating nuclear plant, while Rancho Seco and San Onofre have already shut down. [1] California's grid now has enough solar that the late-afternoon decline in solar output creates a difficult net-demand ramp, while California law still effectively blocks new nuclear construction absent a waste-disposal solution. [1,3,4] Thus, Diablo Canyon's annual production is not merely a large number; it is a measure of how much firm, carbon-free electricity remains from a technology California still uses, still argues about, and has mostly stopped trying to build. [1,5,6]
© Marinos Eliades. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Diablo Canyon Power Plant Operations Assessment Report - 2025 Update," California Energy Commission, CEC-700-2025-005, July 2025.
[2] "Senate Bill 846 Diablo Canyon Power Plant Extension Cost Comparison," California Energy Commission, CEC-200-2023-013-SF, May 2024.
[3] N. Bautista, "Spent Fuel Storage," State of California, Senate Rules Committee, August 2024.
[4] "Report to the Governor on Priority SB 100 Actions to Accelerate the Transition to Carbon-Free Energy," California Energy Commission, CEC-200-2021-008, September 2021.
[5] N. A. Sepulveda et al., "The Role of Firm Low-Carbon Electricity Resources in Deep Decarbonization of Power Generation," Joule 2, 2403 (2018).
[6] J. Aborn et al., "An Assessment of the Diablo Canyon Nuclear Plant for Zero-Carbon Electricity, Desalination, and Hydrogen Production," Massachusetts Institute of Technology, November 2021.
