再エネ出力制御：GW後の振り返りと対策
公開日：2026-10-18
What the 2021 Golden Week Curtailment Data Actually Shows
For solar plant owners in Japan, the Golden Week period is traditionally one of the highest-risk windows for renewable output curtailment (再エネ出力制御). In 2021, there was speculation that curtailment might extend to Hokkaido and Tohoku as well, but in the end, only the Kyushu area actually implemented control. Kyushu Electric Power Transmission and Distribution subsequently updated its curtailment records, making it possible to compare the GW period against 2020.
Looking at the old-rule curtailment counts (旧ルール), the number of control events from April through May 6 was roughly 8–9 in 2020 versus 9–10 in 2021 — essentially unchanged. This matters because the operating methodology changed in 2021 precisely because the previous approach was projected to push old-rule curtailment counts above 30 events. Since the final old-rule count for fiscal 2020 was 22, there is a reasonable likelihood that curtailment in autumn 2021 and March 2022 will exceed the prior year.
One striking detail: more than 20 new projects were commissioned under the old rule between 2020 and 2021. For owners, this is a reminder that the installed base subject to curtailment keeps growing even as the rules shift.
Why the 2021 Rule Change Caught Designated-Rule Operators Off Guard
In April 2021, Kyushu Electric changed how it applies renewable output control, and curtailment frequency under the designated rule (指定ルール) increased noticeably. Operators reported two surprises: control events that never occurred last year now happening on consecutive days, and the shift from rotating (輪番) to uniform (一律) allocation not reducing the curtailment rate per event as expected.
To understand why, it helps to think in terms of a duration curve — a curve that sorts data by magnitude rather than time. When you take a full year of Kyushu demand data (24 hours × 365 days) and reorder it from smallest to largest, the seasonal structure becomes clear. This reframing explains why uniform allocation can produce both more frequent events and higher per-event curtailment rates than operators anticipated.
The practical takeaway for owners is that the designated rule is not simply a milder version of the old rule. Its behavior depends on the shape of the supply-demand balance across the whole year, not on any single day's conditions.
The Structural Driver: Nuclear Restarts, Not Just Sunny Springs
Curtailment occurs when supply exceeds demand, and solar output peaks in spring. But Kyushu's first curtailment began in October 2018, not spring — a signal that the driver was structural. Comparing April and October 2018 during the 12:00–13:00 window, average demand rose 102% while supply rose 108%. Solar and thermal supply actually declined over that period; the increase came from nuclear, with Genkai Units 3 and 4 restarting in May and July respectively.
Nuclear plants must periodically shut down for maintenance and then resume generation once maintenance is complete. Each restart adds baseload supply that does not flex with demand, compressing the headroom available for solar. This is why curtailment risk in Kyushu is tied as much to the nuclear fleet's operating status as to weather.
For owners, this means curtailment forecasting cannot rely on irradiance alone. The supply-side composition — nuclear availability, thermal minimum output, and interconnection capacity — sets the ceiling within which solar must operate.
Will Interconnection Line Reinforcement Reduce Curtailment?
A common assumption is that strengthening interregional interconnection lines (連系線増強) will absorb surplus Kyushu generation and reduce curtailment. The logic is sound in principle: more export capacity means more room for surplus power to flow to other areas. But the effect depends on whether receiving regions have their own surplus or demand headroom at the same moment.
Japan's various utilities have announced possible renewable output control, suggesting that surplus conditions are not unique to Kyushu. If neighboring areas face simultaneous oversupply, additional line capacity delivers less relief than a simple calculation would suggest. Reinforcement helps at the margin, but it is not a substitute for understanding your own plant's exposure.
Owners should therefore treat interconnection upgrades as one variable among several, not as a guaranteed fix.
What This Means for Your Plant Going Forward
The GW 2021 data confirms a pattern: curtailment counts held roughly steady year-over-year during the holiday window, but the underlying rule change and continued new commissioning under the old rule point toward higher curtailment in autumn and the following March. For owners operating under sotsu-FIT (卒FIT) or approaching the 10-year inspection milestone, curtailment directly affects revenue assumptions used in long-term planning.
Rather than reacting to headlines about Hokkaido or Tohoku, the productive step is to establish your plant's actual position: which rule applies to your project, your historical curtailment record by month, and how your output profile aligns with the local supply-demand curve. These facts, not general forecasts, determine your real exposure.
Start with accurately understanding your plant's condition — the applicable rule, the actual curtailment history, and the local grid's structural drivers — before making any decisions about operations, storage, or asset planning.