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Half right. What grows is the cover for wobble. Renewables neither shrink nor grow the cover for a trip.

"The more renewables, the more balancing power" is a familiar explanation. Solar output changes with cloud, wind output with wind, and cover is needed for that. This much is true. But the required volume of balancing power is not determined by that alone.

The requirement has two storeys

OCCTO's formula determines the requirement as the sum of two terms.

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The balancing requirement is the sum of two kinds of coverGround floor: cover for day-to-day wobbleShort-interval wobble in what is left after subtracting solar and wind from demandFrom FY2026, estimated on an ordinary day (1σ) rather than a rare day (3σ)← grows with renewables← can be cut by the estimateFirst floor: cover for a large power station trippingThe single largest station, apportioned by the size of the areaDoes not shrink as renewables grow. Larger where big nuclear units exist← does not shrink
Fig. 1 — The two storeys of the requirement. The ground floor grows with renewables and can also be shrunk by how it is estimated (3σ → 1σ). The first floor is cover for the single largest station tripping, and bears no relation to the volume of renewables. Sources: OCCTO, 44th Balancing Market Subcommittee, Material 2; Agency for Natural Resources and Energy, Material 6, 12 December 2025.

The ground floor is cover for wobble.It estimates statistically how much the residual — demand minus solar and wind output — moves over short intervals. The more renewables, the more that residual moves. This is the substance of "the more renewables, the more is needed."

The first floor is cover for a trip.It is the volume needed to fill the hole when the single largest station stops abruptly. The largest generating unit in eastern Japan was 1,160 MW as of 2021. With Kashiwazaki-Kariwa Unit 6 (1,356 MW) back in 2026, that figure rises. This cover does not shrink however much renewables grow. If anything it grows, the more large units such as nuclear are running.

The 13% cut in FY2026 was to the ground-floor estimate

From FY2026 the ground-floor estimate moved from a band covering even rare days (3σ) to an ordinary-day band (1σ), and tendered volumes for the fast products (primary and secondary I) fell 13% against FY2025. At the same time, tendered volume for the composite product — deliverable for three hours straight — rose 50%. What fell was the ground-floor band; the first floor was left untouched. 47-2 follows the numbers on this.

The same two storeys abroad

Britain's transmission operator NESO has kept its cover for the largest station tripping at the 1,000 MW level, while lowering its minimum inertia floor in steps from 140 in 2022 to 120 (GVA·s) in 2024, and increasing its holdings of fast products instead. Australia's one-second product is likewise set by an inertia-aware formula in which the requirement grows as inertia falls, and its cap was raised from 50 MW in October 2023 to 425 MW in December 2024.

In Britain and Australia, in other words, provision for speed advanced before provision for volume. The ground floor can be shrunk statistically; speed cannot.

How an investor should read this
Ground-floor (wobble) tendered volume moves up and down with the method of estimation. That is the part regulation can shrink. The first floor (cover for a trip) does not shrink, and grows as nuclear returns. In Britain and Australia, provision for speed advanced before provision for volume. Beyond the rise and fall of tendered volumes, the handle on how to take a seat is whether a unit is designed to sit in the three-hour composite product and in the fast products coming next.
The questions in this series
I. Why the grid ran without batteries in the past
  1. 48-2The grid used to run fine without batteries, didn't it?
  2. 48-3What disappears from a grid with fewer spinning machines?
  3. 48-4Why "ten seconds"?
  4. 48-5Does more renewables mean more balancing power is needed? (this article)
II. Why thermal, nuclear and pumped storage are not enough
  1. 48-6Can't thermal just do the balancing?
  2. 48-7If nuclear grows, do we stop needing batteries?
  3. 48-8Wouldn't more pumped storage be enough?
  4. 48-9Where does the balancing power that the market failed to buy come from?
III. Where demand and the generation mix are heading
  1. 48-10Why do data centres point to batteries?
  2. 48-11What happens to solar from here?
  3. 48-12Thermal volume, nuclear operation, demand — what next?
  4. 48-13How much battery capacity is actually coming?
IV. The money, and what the state really wants
  1. 48-14Does the government actually want more batteries?
  2. 48-15What does battery storage resemble as an infrastructure investment?
  3. 48-16They say the earnings will thin out. Is that true?
  4. 48-17Can you copy the overseas playbook and make money in Japan?
  5. 48-18How much battery storage will Japan ultimately need?
  6. 48-19Is a battery the same wherever you put it?
The one question beyond the 1848-20 What do you pay, and what comes back?The answer differs by project, so we have built a calculator whose dials are made only from published figures. You can move unit price, clearing rate, seat fee, arbitrage spread and construction cost yourself.

Sources

Supervised by
Shinya Nakashima(Representative Director, Science X Inc.; Ph.D. in Engineering)

Works on the development, sale and technical due diligence of grid-scale battery storage plants. This column is written and supervised on the basis of hands-on transaction and evaluation practice.