The measured index
The measured index is what a fixed panel of Great Britain's batteries was actually paid, month by month, per kilowatt a year, on the three revenue lines that appear in published files. Every number on it can be rebuilt from public data by anyone with the patience. This page is the recipe.
The panel
Every balancing mechanism registration with any balancing activity, an offer or a bid, or a response and reserve award, in both the first and the last whole month of the window. A registration superseded by a later one at the same site is removed, so a battery that re-registered under a new code is counted once, under the code that carries its later months. The panel is fixed for the year: a battery that arrived in the spring is not in it, and a battery that left is not in it either. That is the point. A fleet average that admits arrivals moves when the fleet changes; this line moves only when the market does.
Three lines, and what each one is
| Line | What is counted | Source | Status |
|---|---|---|---|
| Balancing | Accepted offers, the money paid to discharge, gross. What a battery paid to charge through bids is not deducted, because the energy it bought is sold on in the wholesale market this page cannot see, and deducting the purchase without crediting the sale would charge the Scottish constraint fleet for doing the job it was built for. Many batteries are settled under the unit of the company that trades them, a supplier or aggregator unit, rather than one of their own. Elexon's register links that unit to the battery's grid code, and since 12 October 2026 the battery's actions under it are counted here. Before that date 57 batteries showed no balancing money at all. | Elexon settlement | measured |
| Auctions | Response and reserve awards at their clearing price: award quantity times price times the length of the window. The value the published results imply, not a settled payment. A service that clears below zero is a cost and is counted as one. The index page also prints the year with those days counted at nothing, labelled as such, because Modo Energy's benchmark appears to count them that way and a reader comparing the two should see both. | NESO auction results | measured |
| Capacity market | Each agreement's de-rated megawatts at the price its auction cleared, raised for inflation where the auction set a base period, and split across the year by EMRS's monthly weights. | Capacity Market Register | measured |
| Wholesale trading | Derived, not measured. Elexon publishes each unit's metered output; meter minus accepted balancing volumes, valued at the market index, is the position outside the balancing mechanism. Shown on every page apart from the measured lines. | Elexon B1610 and market index | derived |
The arithmetic
For one battery in one month: the three lines added together, divided by the battery's registered megawatts, multiplied by 365 over the days that month actually has. So a 28 day month and a 31 day month sit on one scale, and a month in progress is annualised on the days so far and moves until the month closes.
The panel line divides the panel's whole income by the panel's whole megawatts, so a big battery counts for more than a small one. The median and the quartiles are taken across batteries, each counting once, which is why the median battery usually sits below the panel line: the large sites earn more per kilowatt than the small ones.
The year figure is the same arithmetic over the twelve months together. Pounds per kilowatt a year multiplied by a thousand is pounds per megawatt a year, the unit the European indices use.
Why a floor, and why that is worth having
Clean Horizon's Storage Index simulates a reference battery, one and a half cycles a day at a stated efficiency, against each month's prices, and reports what it would have earned. Modo Energy's Great Britain benchmark combines settled lines with an estimate of wholesale trading. Both are useful and both are models. This index is the part that is not a model. It is smaller than either, by the wholesale line, and it is the only one of the three a reader can check against a public file.
What we already know is wrong with it
The panel is smaller than the fleet. A battery needs a record in both the first and the last month of the year to be in it, so a battery that arrived during the year is left out however large it is. One large site changing hands or changing optimiser moves the line by a pound or two. The panel and its megawatts are printed beside every figure so the reader can weigh that.
Per kilowatt of what. The rate divides by the balancing register's capacity for the unit. Where the capacity market agreements matched to a site cover more de-rated megawatts than the register gives the unit, the plant earning the money is larger than the registration and the rate is too high. The league page carries the same caveat and marks those rows.
Auction income is implied, not settled. The published results give the award and the price; availability payments after performance adjustments are not public. The auction line is therefore an upper bound on what was actually received for standing ready.
A month in progress is a moving number. The figure for the current month is annualised on the days so far and closes on the first refresh after month end.