The Organization for Cross-regional Coordination of Transmission Operators (OCCTO) says four regions burned through their severe-weather demand assumptions this summer. Its winter outlook nonetheless shows a reserve margin of at least 3% in every area. The findings are in its power supply-demand verification report, dated October 2026.
Summer: the west ran hotter than planned
National peak demand came on 22 July between 14:00 and 15:00 at 164.22 million kW, with a reserve margin of 6.9%. The peak was the second highest in five years and above last year's. In the 16:00 to 17:00 hour that day, when solar output fades, the margin fell to 5.6%. That is 5.3 points below the May outlook, and supply capacity was 14.32 million kW short of it.
At the peak hour, supply across the 10 areas was 18.47 million kW below the May outlook. The outlook figure does not allow for unplanned outages. Unplanned outages ran at 2.5% of supply on the peak day and 2.1% on average from July to August. The verification had assumed 2.6%.
Chubu, Chugoku, Shikoku and Kyushu all exceeded their summer severe-weather (H1) demand assumption. Chubu peaked at 25.71 million kW on 22 July against an assumed 25.03 million kW. Kyushu reached 17.09 million kW on 3 August against 16.75 million kW. Chugoku (10.80 million kW) and Shikoku (5.02 million kW) also topped their assumptions on 3 August. Demand beat the severe-weather assumption on 15 days in total, 14 of them in Chubu through Kyushu and seven in Chugoku.
In Hokuriku, the reserve margin at the area's own peak was below 3%, while the national margin at the same hour was 6.9%. OCCTO says it will work with the grid operators in the four areas to reflect this summer's actuals in demand assumptions for next summer's verification.
Winter: thinnest in January
OCCTO tests whether supply covers at least 103% of demand under weather as severe as the harshest of the past 10 years, both at peak demand and at the minimum-reserve hour in the evening and night. With dispatch-command capacity, thermal output increases and transfers between areas counted, every area clears the 3% minimum. The 10-area margin is tightest in the first half of January. In that period seven of the nine areas excluding Okinawa sit at 4.5% at peak demand, while Hokkaido has 12.5% and Shikoku 14.4%.
| Period | Margin at peak demand (%) | Margin at minimum-reserve hour (%) |
|---|---|---|
| Early December | 7.5 | 7.3 |
| Late December | 12.6 | 12.1 |
| Early January | 5.3 | 5.2 |
| Late January | 6.6 | 6.9 |
| Early February | 7.3 | 7.1 |
| Late February | 8.0 | 7.5 |
| Early March | 9.3 | 9.7 |
| Late March | 10.7 | 10.6 |
Hokkaido's cable constraint
OCCTO also tested a large loss of generation capacity. In the second half of January, a 1.29 million kW loss, the largest generation outage in Hokkaido in the past 10 years, would still leave a 5.6% reserve against severe-cold demand. That result relies on power received from other areas over the Hokkaido-Honshu interconnector and on contracted demand curtailment. This winter the interconnector's operating capacity drops from 0.9 million kW to 0.6 million kW because of works on the new second pole. OCCTO says the government, grid operators and OCCTO will coordinate to maintain supply.
The winter figures are projections. OCCTO notes that unplanned generator outages and other shifts in supply can occur in real time. It plans to issue supply-provision notices when the wide-area reserve margin falls below 8%, starting at the weekly-plan and day-after-next stages, and to monitor both kW and kWh through the winter. It is preparing to alert the public through social media if supply looks tight.