If nuclear grows, do we stop needing batteries?

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The opposite. Because nuclear does not throttle down at midday, the midday surplus grows and so does the work for batteries.

"Once nuclear comes back, we'll have enough electricity, and batteries won't be needed." It is a common view, but the grid works the other way round. The reason is that nuclear is a resource that does not throttle down at midday.

The curtailment order is fixed, and nuclear is last

At midday on a sunny holiday, when solar exceeds demand, something has to be curtailed. The order is set by national rules (the priority dispatch rules). Thermal is squeezed to minimum first; next, pumped-storage charging and transfers to other areas absorb what they can; if surplus remains, biomass is curtailed, then solar and wind. Hydro, nuclear and geothermal come last. The reason, as the Agency for Natural Resources and Energy explains, is that finely adjusting their output over short intervals is technically difficult, and once brought down they cannot be brought back quickly.

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Fig. 1 — The curtailment order when electricity is in surplus. Nuclear comes last as a long-term fixed resource, and in practice does not come down at midday either. Source: Agency for Natural Resources and Energy, "Naruhodo! Grid — On Output Curtailment."

What is happening in Kyushu

Kyushu has four reactors totalling 4.14 GW: Sendai 1 and 2 (890 MW each) and Genkai 3 and 4 (2.36 GW combined). Apart from periodic inspection outages, these four run at the same output day and night. Layer on the densest solar fleet in Japan, and on a sunny midday generation exceeds consumption. Nuclear does not come down, so what is curtailed is solar. Kyushu's curtailment rate for solar and wind hit a record 8.3% (1.29 TWh) in FY2023, was 4.8% (750 GWh) in FY2024, and is forecast at 6.1% for FY2025 — the highest in the country throughout.

Take an actual day. At 12:30 on 4 May 2025, Kyushu demand was 6.53 GW. Solar was producing 9.68 GW, and nuclear, hydro and geothermal 3.34 GW. Even after pumping 1.89 GW uphill, exporting 1.34 GW to other areas and storing 40 MW in batteries, a surplus remained, and 5.09 GW of solar and wind was curtailed (Agency for Natural Resources and Energy, 3rd Next-Generation Power Grid WG, Material 1, 27 June 2025, p.6). Nuclear did not come down that day. In OCCTO's FY2031 grid simulation, the 17 reactors restarted or expected to restart are likewise modelled as must-run at constant output year-round (103rd Wide-Area Grid Development Committee, Material 3, p.52).

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Fig. 2 — Midday on a sunny holiday. Nuclear does not throttle down by day, and thermal squeezed to minimum still cannot be stopped entirely. Layer solar on top and midday generation exceeds consumption, with the excess met by curtailing solar. A battery stores that surplus and delivers it in the evening (conceptual; Kyushu's actuals are in the body text).

Curtailed electricity need not be thrown away if somebody stores it. Storing cheaply at midday and selling into the expensive evening hours is a battery's job. The more nuclear runs, the larger the midday surplus and the more of that work there is. Agency for Natural Resources and Energy materials list the resumption of commercial nuclear operation as one of the factors behind rising curtailment in the Chugoku area in FY2024.

Where, and when, does nuclear come back?

As of April 2026, 15 reactors are operating: four in Kyushu (Sendai 1 and 2, Genkai 3 and 4), seven in Kansai (Takahama 1–4, Ohi 3 and 4, Mihama 3), one in Shikoku (Ikata 3), one in Tohoku (Onagawa 2, November 2024), one in Chugoku (Shimane 2, December 2024) and one in Tokyo (Kashiwazaki-Kariwa 6, February 2026). The areas where the midday surplus is being pushed up by nuclear are therefore Kyushu, Kansai, Shikoku, Tohoku, Chugoku and Tokyo. Chubu and Hokuriku have no operating reactors.

Next to return are Hokkaido and Tokyo. Tomari 3 in Hokkaido (912 MW) received its installation-change permit in July 2025, and the prefectural governor consented to the restart in December (the operator is targeting a restart as early as possible in 2027). Hokkaido's minimum midday demand in spring and autumn is 2.81 GW (6 May 2025, Agency for Natural Resources and Energy), so a restarted Tomari 3 would on its own make roughly a third of minimum demand a resource that never comes down. Hokkaido's long-term curtailment-rate outlook is 30%, the highest in the country (48-11). In Tokyo, Kashiwazaki-Kariwa 7 (1,356 MW) sits behind Unit 6, and the prefectural governor approved the request for understanding on Units 6 and 7 in December 2025. For Tokai Daini (1,100 MW), the operator has disclosed that safety works are not expected to be completed by December 2026 (Japan Atomic Power Company, January 2026, reported).

Nuclear is set to grow from here

The same thing happened in Tokyo. On 21 January 2026, Kashiwazaki-Kariwa 6 (1,356 MW) restarted after about fourteen years (it halted briefly on a fault, restarted on 9 February, began transmitting on 16 February and entered commercial operation on 16 April). Two weeks after transmission began, on 1 March, solar was curtailed in the Tokyo area for the first time. TEPCO Power Grid's instructions ran from 30 MW to 1.18 GW by block, and the published total curtailed volume was 1.84 GW (1.81 GW solar and wind plus 40 MW biomass) (TEPCO PG published figures; Nikkei BP, March 2026 — the two use different aggregation methods). TEPCO PG cited low holiday demand as the reason for the scale. The 7th Strategic Energy Plan (February 2025) sets FY2040 nuclear at around 20% and drops the phrase about reducing dependence as far as possible. The FY2024 actual was 9.4%. On the premise that nuclear's share doubles, midday surpluses grow across the country.

Some countries do have nuclear perform midday balancing. In France, nuclear supplies about 70% of electricity (68.1% in 2025, per the transmission operator RTE), and many reactors run at reduced output during spring and summer days. Japan's reactors do not operate that way, for reasons of regulation and operating convention. So in Japan, the more nuclear grows, the more batteries are needed to absorb the midday surplus.

How an investor should read this

The regions where nuclear returns are the regions where the midday surplus grows. Midday is currently being pushed up by nuclear in Kyushu, Kansai, Shikoku, Tohoku, Chugoku and Tokyo; next to be pushed up are Hokkaido (Tomari 3, local consent obtained) and Tokyo (Kashiwazaki-Kariwa 7). Kyushu, with four reactors and the densest solar fleet in Japan, has the highest curtailment rate in the country; Hokkaido's long-term outlook is 30%. Curtailed electricity is electricity that can be stored cheaply, and it is the raw material of a battery's arbitrage spread. The FY2040 premise of around 20% nuclear works in the direction of a larger midday surplus.

The questions in this series

I. Why the grid ran without batteries in the past

II. Why thermal, nuclear and pumped storage are not enough

48-6Can't thermal just do the balancing?
48-7If nuclear grows, do we stop needing batteries? (this article)
48-8Wouldn't more pumped storage be enough?
48-9Where does the balancing power that the market failed to buy come from?

III. Where demand and the generation mix are heading

IV. The money, and what the state really wants

The one question beyond the 1848-20 What do you pay, and what comes back?The answer differs by project, so we have built a calculator whose dials are made only from published figures. You can move unit price, clearing rate, seat fee, arbitrage spread and construction cost yourself.

Sources

Agency for Natural Resources and Energy, "Naruhodo! Grid — On Output Curtailment" (priority dispatch rules; nuclear last)
Agency for Natural Resources and Energy, "Short-Term Outlook for Renewable Output Curtailment," 23 January 2025 (Kyushu FY2023: 8.3%, 1.29 TWh); 8th Next-Generation Power Grid WG, Material 1, 16 March 2026 (Kyushu FY2024: 4.8%; FY2025 forecast 6.1%)
Kyushu Electric Power (Sendai 1 and 2, Genkai 3 and 4)
Agency for Natural Resources and Energy, "Recent Developments in Nuclear Policy," 5 June 2026, Material 1 (Kashiwazaki-Kariwa 6: started 21 January 2026, restarted 9 February, generating and transmitting 16 February, commercial operation 16 April); TEPCO Power Grid press release, 1 March 2026 (instructions of 30 MW–1.18 GW); Nikkei BP Mega Solar Business, March 2026 (total curtailed volume 1.84 GW, TEPCO PG published figure)
Agency for Natural Resources and Energy, 3rd Next-Generation Power Grid WG, Material 1, 27 June 2025 (supply-demand balance in Kyushu on the minimum-demand day, 4 May 2025); OCCTO, 103rd Wide-Area Grid Development Committee, Material 3, 1 September 2026 (17 reactors modelled as must-run year-round)
Kyushu Electric Power, overview of the Genkai Nuclear Power Station (Units 3 and 4, 2.36 GW combined); Kagoshima Prefecture, "Overview of Sendai Nuclear Power Station Units 1 and 2" (licensed output 890 MW)
Agency for Natural Resources and Energy, "Recent Developments in Nuclear Policy," 31 March 2026, Material 1, and 5 June 2026, Material 1 (operating status; Tomari 3 permit and consent; the governor's approval on Kashiwazaki-Kariwa 6 and 7; construction status at Tokai Daini); Japan Atomic Industrial Forum, "Nuclear Power Reactors in Japan," as of 9 March 2026 (list of operating reactors); Hokkaido Prefecture, "Background to the Consent Decision on the Restart of Tomari Unit 3," 10 December 2025; press reports of Japan Atomic Power Company's announcement, January 2026 (completion timing of Tokai Daini works); Agency for Natural Resources and Energy, 3rd Next-Generation Power Grid WG, Material 1, 27 June 2025, p.6 (Hokkaido minimum-demand day, 2.81 GW)
7th Strategic Energy Plan, Cabinet decision of 18 February 2025 (FY2040 nuclear around 20%); METI, "FY2024 Energy Supply and Demand Results (final)," 14 April 2026 (FY2024 nuclear 9.4%)
Agency for Natural Resources and Energy, Grid WG, 50th meeting (resumption of commercial nuclear operation among the factors increasing FY2024 curtailment in the Chugoku area)
RTE, "Bilan électrique 2025" (France's nuclear share 68.1%; daytime output reduction); World Nuclear Association (general description of load following)

Supervised by

Shinya Nakashima(Representative Director, Science X Inc.; Ph.D. in Engineering)

Works on the development, sale and technical due diligence of grid-scale battery storage plants. This column is written and supervised on the basis of hands-on transaction and evaluation practice.