Tokyo’s power grid, like all other transmission system operators in Japan, has faced economic constraints as growth in solar power production outpaces the flexibility of the country’s largest regional electricity market.
Tepco power grid instructed sustainable producers to reduce production on March 1, 2026, from 11:00 AM (JST) to 4:00 PM, citing high solar power generation, low weekend demand and zero available inter-regional export capacity. It was the first time the utility had ever ordered economic containment.
“The start of curtailment in the Tepco area is of great significance because it shows that even in Japan’s largest energy market – Tokyo, historically a net-importing load center – renewable energy deployment is starting to outpace system flexibility,” said Michiyo Miyamoto, energy finance specialist for Japan at the Institute for Energy Economics and Financial Analysis (IEEFA).
Production increased by no less than 1.84 million kW at the peak. Multiple Japanese-language outlets confirmed the event as Tepco’s first-ever economic curtailment – distinct from a local grid curtailment at the Boso substation in Chiba Prefecture on January 6, 2025, which only affected producers with non-fixed grid connections.
The actual containment followed on March 8 and 21, with three more dates announced in advance but not implemented. The March 21 event was particularly significant: it occurred without any online nuclear generation, meaning the solar energy alone was enough to overwhelm the system flexibility in Tokyo’s power grid.
Every other transmission system operator area (TSO) in Japan had already experienced economic curtailment before March 2026. The containment was limited to Kyushu through fiscal 2021, spread to Hokkaido, Tohoku, Chugoku, Kansai, Shikoku and Okinawa in fiscal 2022, and reached Chubu and Hokuriku in fiscal 2023, according to April 2024 data published by the Tokyo-based Institute for Renewable Energy (REI), citing the Ministry of Economy, Trade and Industry (METI).
Tepco’s service area was the only exception. Japan’s grid operator, the Organization for Cross-regional Coordination of Transmission Operators (OCCTO), said in its 2024 budget outlook that there were curtailments in nine areas – Tokyo was not among them. A METI working group predicted in early 2025 that Tokyo would experience its first economic contraction in fiscal year 2025, with a forecast rate of 0.009%.
“This indicates that containment is no longer a regional issue limited to areas like Kyushu, but a nationwide structural challenge,” said IEEFA’s Miyamoto. pv magazine.
Miyamoto said system flexibility has not kept pace with the integration of renewable energy sources. Thermal generation cannot be reduced sufficiently during periods of oversupply; operational constraints typically limit output to 30% to 50% of maximum, depending on the type of installation. Pumped hydroelectric power stations are already in intensive use, and battery energy storage systems (BESS) remain too limited to absorb the surplus. Containment has effectively become the default balancing mechanism, she said.
Solar and wind curtailments across Japan totaled 1.74 TWh in the first half of 2025, a record six-month period, exceeding METI forecasts for the entire fiscal year 2025 in several areas, according to Japan Energy Hub. In October 2025, Reuters reported that the annual curtailment was on track to reach record levels, partly because nuclear units – which have priority for shipment – were generating more.
Miyamoto said the most immediate remedies are expanded battery storage, stronger inter-regional transmission and improved market incentives. Progress on all three fronts is being slowed by grid connection bottlenecks and market design constraints, including capped market prices and limited auction volumes in the long term, she said. Policy frameworks have tended to accommodate containment – through mechanisms such as non-fixed connectivity rules – rather than reduce it.
The March 21 curtailment, Miyamoto said, “took place even without nuclear power generation, indicating that the curtailment is no longer an isolated or one-time event, but an emerging structural problem.”
Miyamoto is the author of an August 2025 IEEFA report on barriers to Japan’s renewable energy development. Representatives from the Institute for Sustainable Energy Policies (ISEP), BloombergNEF and Rystad Energy were contacted for this story and had not responded before publication.
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