From the 1 September 2026 delivery, the primary offline cap became ¥10.00/ΔkW·30min. This article counts where primary offline bids landed before and after that, using the trading results of EPRX (the Electric Power Reserve Exchange). The one question: in September, with the cap at ¥10, how far did battery bids fall, and where did they hold?
There are two windows: 1 April – 31 August under the ¥15 cap (153 days), and 1–29 September under the ¥10 cap (29 days). Both use EPRX preliminary figures; areas are by where the resource is located, and prices are volume-weighted averages (Σ average awarded price × awarded volume ÷ Σ awarded volume). How the cap came down, and how much it removes, is in COLUMN 41.
1. Nationally, it halved
← Scroll sideways →
The national weighted average for primary offline went from ¥11.07 in April–August to ¥5.81 in September. On final figures April–August is ¥11.08, a difference of ¥0.01 from the preliminary figures. Final figures for September will be published by the end of October.
| Area | Apr | May | Jun | Jul | Aug | Sep |
|---|---|---|---|---|---|---|
| Tohoku | 13.49 | 14.86 | 14.66 | 14.49 | 14.52 | 9.51 |
| Tokyo | 11.47 | 11.59 | 12.09 | 11.76 | 11.42 | 8.58 |
| Chubu | 14.57 | 14.34 | 13.15 | 10.25 | 9.08 | 3.69 |
| Kyushu | 11.94 | 10.87 | 6.89 | 5.67 | 4.97 | 3.04 |
| National | 12.29 | 12.53 | 11.71 | 10.25 | 9.63 | 5.81 |
Nationally, from ¥12.53 in May the price had already fallen into the ¥10 range in July and the ¥9 range in August, before the cap came down. Chubu and Kyushu fell sharply over the summer and dropped another step in September. Tohoku and Tokyo did not move until August and came down with the cap on 1 September. The national figure is the sum of these two different movements.
2. A split in two
← Scroll sideways →
| Area | Apr–Aug | Sep | Award rate (Sep) |
|---|---|---|---|
| Hokkaido | 5.50 | 1.39 | 77.2% |
| Tohoku | 14.51 | 9.51 | 95.6% |
| Tokyo | 11.67 | 8.58 | 85.1% |
| Chubu | 11.75 | 3.69 | 45.6% |
| Hokuriku | 9.38 | 3.19 | 66.0% |
| Kansai | 11.67 | 3.26 | 62.6% |
| Chugoku | 10.02 | 3.34 | 48.5% |
| Shikoku | 5.92 | 2.74 | 33.8% |
| Kyushu | 7.54 | 3.04 | 53.9% |
| National | 11.07 | 5.81 | 63.2% |
Tohoku and Tokyo stayed just under the cap. In Tohoku, bids of ¥10 or more were awarded on all 29 days of September — 1,260 slots in all. Only 21 slots, on 1 September, were displayed above ¥10; the rest were exactly ¥10.00, pinned to the cap. Tokyo came in at ¥8.58, with ¥10 or more on 11 days.
The other seven areas fell to the ¥1–3 range. Chubu went from ¥11.75 to ¥3.69, Kansai from ¥11.67 to ¥3.26. In the month the cap fell to ¥10, they were awarded more than ¥6 below it.
The displayed maximum awarded price also changed in September. In April–August the maximum reached ¥15.00 in all nine areas. In September the only displays above ¥10 were ¥15.00 in Tohoku and ¥12.50 in Kansai, both on 1 September. EPRX's system does not reject bids above the cap; it cuts them to the cap at settlement, so what is received is ¥10.00. From the second day on, no display above ¥10 has appeared.
3. Why it split — the award rate
← Scroll sideways →
If one number separates the two groups, it is the award rate: the share of bids that were awarded. Tohoku is at 95.6% and Tokyo at 85.1%. In areas where almost every bid gets through, the price leans towards the cap. In areas where about half the bids fail — Chubu 45.6%, Chugoku 48.5%, Kyushu 53.9% — the price is in the ¥3 range.
Tohoku was at ¥14.51 in April–August too, close to the ¥15 cap. Bid volume was 0.28 times the procurement volume — a market where bids do not reach even 30% of what is sought. In September it rose to 0.62 times, but still falls short. In a market that is short, there is no reason to cut bids, and when the cap comes down the price comes down by just that much. Tohoku's ¥9.51 is that shape. In Tokyo, bids exceed the procurement volume (1.16 times), yet 85% of bids still get through.
4. Hardly any change by time of day
← Scroll sideways →
| Area | 0:00–4:00 | 4:00–8:00 | 8:00–12:00 | 12:00–16:00 | 16:00–20:00 | 20:00–24:00 |
|---|---|---|---|---|---|---|
| Tohoku | 9.58 | 9.54 | 9.51 | 9.43 | 9.52 | 9.49 |
| Tokyo | 8.50 | 8.50 | 8.59 | 8.57 | 8.63 | 8.64 |
| Chubu | 3.34 | 3.39 | 4.34 | 4.13 | 3.48 | 2.92 |
| Kyushu | 2.77 | 2.77 | 3.13 | 3.33 | 3.28 | 2.77 |
| National | 5.68 | 5.76 | 5.95 | 5.90 | 5.82 | 5.70 |
Nationally, prices stay between ¥5.68 and ¥5.95, and Tohoku is at ¥9.58 even in the small hours, 0:00–4:00. By contrast, what a battery can earn on JEPX (the wholesale market) varies a great deal by time of day: ¥5.8–8.5 for the 8 evening slots depending on the area, and ¥1.0–1.4 averaged over 48 slots (47-4 Floor).
The guideline's ceiling is “opportunity cost + fixed-cost adder + trading fee”. How to count a battery's opportunity cost is not settled in the text (unsettled), but on a view close to “what you fail to earn on the wholesale market”, night-time opportunity cost is small. The adder set from published figures is in the ¥1 range even in year 1 (50-3 Calculation). Explaining a ¥9 bid in the middle of the night with the formula is not easy. Where the second ceiling bites hardest is on night-time bids in the areas where prices are pinned to the cap.
5. How to read the numbers
- These are preliminary figures. Final figures for September will be published by the end of October; when they are, we will replace this article's September numbers with them. For April–August, preliminary and final figures differed by no more than ¥0.03.
- These are displayed values before settlement. EPRX's system does not reject bids above the cap; it cuts them to the cap at settlement. In September's primary offline, the only displays above ¥10 were 21 slots in Tohoku and 2 in Kansai, both on 1 September.
- Some areas are thin markets. In Shikoku in September, 880 of 1,392 slots had no award; in Hokuriku it was 314 slots. Those areas' averages come from correspondingly fewer slots.
- It is 29 days. This is the first month under the ¥10 cap, and seasonal differences cannot be seen yet.
6. For owners of 2 MW / 8 MWh
Within the same primary offline, September's position split in two by area. In Tohoku and Tokyo, the ¥10 market cap is the ceiling as it stands. In the other seven areas prices sit more than ¥6 below the cap, and competition sets the price before any ceiling does.
In both groups of areas, the guideline's second ceiling has not gone away. Where prices are pinned to the cap, even bids in the middle of the night are being awarded close to ¥10. Whether that can be explained by opportunity cost and the adder is exactly what will be asked when the Commission checks high-priced awards after the fact (50-5 Monitoring). For the unit price in a business plan, the safe choice is not the ¥10 cap but the lower of your area's actual September price and the height to which your own bids can be explained by the formula.
Sources
- EPRX, trading results (primary offline results, preliminary and final) https://www.eprx.or.jp/information/results.php (retrieved 29 September and 4 October 2026; deliveries 1 April – 29 September 2026)
- EPRX, “On the ΔkW price cap in the balancing market” (30 July 2026) https://www.eprx.or.jp/information/docs/14b85fbe8942da7d3bdddda80243e7e170f64d9e.pdf
- METI, “Balancing Market Guideline” (需給調整市場ガイドライン), revised 13 March 2026 https://www.egc.meti.go.jp/info/guideline/pdf/20260313001a.pdf
The aggregation formulas, CSV field names and full result tables are in the documentation of our public aggregation script (scripts/eprx/README.md in the scix-web repository).