← Back to the entry point (all 19 questions)

Because frequency falls within seconds. The fewer the spinning machines, the faster it falls, and the faster the fill has to be.

When the largest power station stops abruptly, generation falls short in that instant and frequency starts to drop. What slows the drop is the momentum of the spinning machines. On a grid with plenty of momentum the fall takes tens of seconds; on a grid with little, it takes a few.

Somewhere during that fall, somebody has to fill the hole. Fill it too slowly and frequency drops too far, triggering the devices that automatically cut supply to homes and factories. That is why "within how many seconds must it be filled" becomes the defining condition of a balancing product.

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How frequency falls immediately after a power station trips50Hz↓ fall this far and demand is cut automatically0 sec1 sec10 sec20 secMany spinning machines: slow fallFew spinning machines: falls within secondsFilled in 1 secondFilled in 10 secondsConceptual. Vertical axis: extent of frequency drop. Horizontal axis: seconds since the station tripped
Fig. 1 — How frequency falls immediately after a power station trips (conceptual). With many spinning machines the fall is slow and filling in ten seconds is enough. With few, it is fast, and a product that fills within one second is needed.

Ten seconds in Japan, one second abroad

The condition on Japan's fastest product (primary control reserve) is to sense the frequency drift itself, begin responding within ten seconds, and sustain for at least five minutes (Agency for Natural Resources and Energy, Material 8, 13 May 2026). This product is served by batteries and thermal, and it has a bracket only batteries can sit in (offline) and a bracket where they compete with thermal (online) — see 47-3.

Abroad, faster products were built. Britain has run a product delivering full volume within one second (Dynamic Containment) since October 2020, and Australia a one-second product (Very Fast FCAS) since October 2023. Texas requires 15 cycles — 0.25 seconds.

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Time allowed to fill the whole hole left by a tripped stationTexas, USA0.25 secfrom March 2020Britain1 secfrom October 2020Australia1 secfrom October 2023Japan10 sec
Fig. 2 — Time allowed to fill the whole hole when a large power station stops. Texas: ERCOT Fast Frequency Response (15 cycles). Britain: NESO Dynamic Containment. Australia: AEMO Very Fast FCAS. Japan: primary control reserve in the balancing market.

Why Britain built a one-second product

On 9 August 2019, a lightning strike in Britain was followed by the successive loss of offshore wind and gas-fired generation, taking out 1,481 MW in total. The system was secured against a 1,000 MW loss. Frequency fell to 48.8 Hz, roughly 1 GW of demand was cut automatically, and more than a million customers lost power. Frequency took 4 minutes 42 seconds to return to normal, and full restoration took about 45 minutes. In October 2020 the transmission operator launched its one-second product, on the grounds that system inertia had reached an all-time low and frequency was departing faster, requiring a faster response. The first auction selected two battery sites totalling 90 MW.

Fast products are coming to Japan too

Ten seconds is the entry point.Britain built its one-second product in 2020 and Australia in 2023, and in both cases batteries became the main providers. Turning inertia itself into a product is also advancing in Britain, through procurement of synchronous condensers and grid-forming capability (Stability Pathfinder). Momentum will decline on Japan's grid too. In July 2024 OCCTO set out an approach of managing the rate of frequency fall to within 2.0 Hz per second, and published an estimate that six areas of western Japan would exceed that reference by 2050. Work on turning batteries that can imitate momentum (grid-forming) into a product is also advancing in the Grid Code Study Group. Neither the one-second product nor momentum is yet a product in Japan. When they become one, two more seats open up for batteries.
How an investor should read this
Among non-spinning resources, batteries are the easiest to supply fast products from. Japan's ten-second product already has its battery-only offline bracket half filled (47-3). The next two — one second and momentum — are not yet products in Japan. In Britain and Australia, every time a faster product was created, the work available to batteries grew. Both are seats batteries are well placed to take, though depending on the requirements, control-system modifications may be needed. The regulatory work is moving in the direction of putting a price on speed and on momentum.
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"? (this article)
  4. 48-5Does more renewables mean more balancing power is needed?
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.