How we work out capacity market income per site
The capacity market pays plant to exist, so that the country has enough of it available on the worst winter evening. It is the third of the four ways a battery earns, and unlike trading it is published. This is how we turn what is published into a figure per site, and the four places where it can go wrong.
What is published, and where
| Input | Source | Status |
|---|---|---|
| Which units hold an agreement, and for which years | Capacity Market Register | measured |
| De-rated capacity each agreement pays on | Capacity Market Register | measured |
| The price each auction cleared at | NESO auction results | measured |
| Which agreements have been terminated | Capacity Market Register | measured |
| Where the site physically is | Ordnance Survey grid reference | measured |
| Which mapped battery an agreement belongs to | nowhere | matched |
The calculation
- Take every agreement marked as a storage facility, awarded, and not terminated.
- Drop pumped hydro. More on that below, because it is the trap.
- Read the delivery years each agreement is valid for. A fifteen year agreement awarded years ago is still being paid now.
- Price it at the auction that awarded it, not at the current auction. An agreement from 2020 is paid the 2020 price, raised for inflation.
- Raise that price for inflation where the auction set a base period, which the four and three year ahead auctions do. The rule is in Schedule 1 of the Electricity Capacity Regulations 2014: the cleared price times the average consumer price index for October to April before the delivery year, divided by the same average for the base winter. The index is the Office for National Statistics' CPI, series D7BT. One year ahead agreements are not raised.
- Multiply the de-rated obligation in kilowatts by that price.
- Split the year by month. Payments follow electricity demand, so a January pays about a quarter more than an even twelfth. EMRS publishes the weight for each month. A delivery year runs October to September and our window does not, so each month in the window is counted at its weight, and a part month for the days it covers.
The trap, and how much it was worth
The register marks pumped hydro as a storage facility, which it is. It is not a battery, and it is enormous. Six Dinorwig units at 274 megawatts each, four at Cruachan, two at Foyers, two at Ffestiniog.
Our first run of this calculation returned £140m at the cleared price. Taking pumped storage out, on the same basis, removed more than half of it: more than half the number was somebody else's mountain. With inflation uprating and the monthly weights now applied, the battery figure for the current window is £78.8m. Great Britain has four pumped storage stations, so they are excluded by name rather than by a rule, and the names are in the code where anyone can check them.
Four things we know are wrong with it
One. This is what the rules say is paid, not a settled payment. The inflation uprating and the monthly weights follow the regulations and EMRS's published figures, so the result should match what a holder is paid before any penalty or adjustment. Until 25 September 2026 we published the cleared price spread evenly, because we had not yet checked the uprating rule. That figure was about a fifth below the real payment. The rule has now been read from the regulations and applied.
Two. A third of the money cannot be attached to a mapped site. The capacity market register and the balancing data share no identifier at all, so the match is on coordinates, corroborated by name. Within 900 metres the coordinate carries it. Beyond that, up to five kilometres, it needs a distinctive shared word, and "energy" or "limited" is not distinctive. Of £78.8m, we can place £61.3m on 157 sites. The rest is described below rather than quietly dropped.
Three. Payments are not guaranteed. An agreement can be suspended, and capacity can be traded away between parties. We read the termination flag and drop terminated agreements, but we do not yet follow secondary trading, so an obligation that has changed hands is still credited to the original holder.
Four. De-rating is not duration. The de-rated capacity is what the agreement pays on, and it reflects how long a battery can run as much as how big it is. It is not the site's power rating and should not be read as one.
What the unmatched money turned out to be
Most of it is not a matching failure. It is 35 battery sites holding 389 megawatts of de-rated agreements and taking £9.0m that appear in neither the balancing mechanism nor the reserve auctions. Nothing else about them is published, so they are not on our map. The capacity market is the only place they show up at all.
That is worth saying plainly: our fleet of 250 assets is not the whole fleet, and the capacity market tells us roughly by how much.
Tell us we are wrong
If you hold one of these agreements and our figure for your site is wrong, or we have attached it to the wrong place, that is the most useful message we could get. Say which unit and what it should be.