Tokyo’s grid has joined every other transmission system operator area in Japan in experiencing economic curtailment, as solar output growth outpaces the flexibility of the country’s largest regional power market.
Tepco Power Grid instructed renewable generators to reduce output on March 1, 2026, from 11:00 (JST) to 16:00, citing high solar generation, low weekend demand, and zero available inter-regional export capacity. It was the first time the utility had ever ordered economic curtailment.
“The onset of curtailment in the Tepco area is highly significant because it shows that even in Japan’s largest power market – Tokyo, historically a net-importing load center – renewable deployment has begun to outpace system flexibility,” said Michiyo Miyamoto, energy finance specialist for Japan at the Institute for Energy Economics and Financial Analysis (IEEFA).
Output was cut by as much as 1.84 million kW at peak. Multiple Japanese-language outlets confirmed the event as Tepco’s first-ever economic curtailment – distinct from a localized grid curtailment at the Boso substation in Chiba prefecture on Jan. 6, 2025, which affected only generators with non-firm grid connections.
Actual curtailment followed on March 8 and March 21, with three further dates announced in advance but not implemented. The March 21 event was particularly significant: it occurred with no nuclear generation online, meaning solar output alone was sufficient to overwhelm system flexibility in the Tokyo grid.
Every other transmission system operator (TSO) area in Japan had already experienced economic curtailment before March 2026. Curtailment was confined 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 Renewable Energy Institute (REI), citing the Ministry of Economy, Trade and Industry (METI).
The Tepco service area was the sole exception. Japan’s grid operator, Organization for Cross-regional Coordination of Transmission Operators (OCCTO), said in its fiscal 2024 outlook that there was curtailment in nine areas – Tokyo was not among them. A METI working group forecast in early 2025 that Tokyo would see its first economic curtailment in fiscal 2025, with a projected rate of 0.009%.
“This signals that curtailment is no longer a regional issue confined to areas like Kyushu, but a nationwide structural challenge,” IEEFA’s Miyamoto told pv magazine.
Miyamoto said that system flexibility has not kept pace with renewable integration. Thermal generation cannot be reduced sufficiently during periods of excess supply – operational constraints typically limit output to 30% to 50% of maximum depending on plant type. Pumped hydro is already heavily utilized, and battery energy storage systems (BESS) remain too limited to absorb the surplus. Curtailment has effectively become the default balancing mechanism, she said.
Solar and wind curtailment across Japan totaled 1.74 TWh in the first half of 2025, a record for any six-month period, with multiple areas exceeding METI’s full-year fiscal 2025 forecasts, according to Japan Energy Hub. By October 2025, Reuters reported that annual curtailment was on track to hit record levels, in part because nuclear units – which hold dispatch priority – were generating more.
Miyamoto said the most direct 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 balancing market prices and limited long-term auction volumes, she said. Policy frameworks have tended to accommodate curtailment – through mechanisms such as non-firm connection rules – rather than reduce it.
The March 21 curtailment, Miyamoto said, “occurred even in the absence of nuclear generation, indicating that curtailment is no longer an isolated or one-off occurrence but an emerging structural issue.”
Miyamoto is the author of an August 2025 IEEFA report on barriers to Japan’s renewable energy development. Representatives of the Institute for Sustainable Energy Policies (ISEP), BloombergNEF, and Rystad Energy were contacted for this story and had not responded by publication time.
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