Two equally busy batteries, and one follows the price fifteen times more closely
Everybody assumes a battery charges when power is cheap and sells when it is dear. It is the whole premise of the asset class. It is also checkable, half hour by half hour, and the answer is that some sites do it far more tightly than others. We cannot explain why, and that is the interesting part.
What was measured
For twenty one days we recorded where every battery in the balancing mechanism actually was, half hour by half hour. Not what it said it would do: the level it was instructed to run at where the control room had instructed it, and the level it notified where it had not. Reading notifications alone gives the wrong answer, and we have written up why.
Then each site's position was set against the wholesale reference price in the same half hour. A site that sells hard into the dear half hours scores high. A site whose position has nothing to do with the price scores near zero.
The first thing it measures is whether a site moves at all
Twelve sites moved in fewer than 15 per cent of half hours over three weeks. They score near zero, and that says nothing about how well they are traded, only that they were not doing much. So they are set aside.
That leaves 66 sites, 3,088 megawatts, moving in at least 40 per cent of half hours. Among those the spread is wide and it is real.
Fifteen times the difference, between equally busy sites
| Site | Trader | MW | Moving | Score |
|---|---|---|---|---|
| BURWB-3 | EDF ENERGY CUSTOMERS LIMITED | 24 | 59% | +0.45 |
| T_KILSB-2 | ZENOBE KILMARNOCK SOUTH LTD | 52 | 49% | +0.43 |
| T_KILSB-4 | ZENOBE KILMARNOCK SOUTH LTD | 52 | 57% | +0.43 |
| KXP Immingham BESS | KXP Immingham Ltd | 81 | 52% | +0.40 |
| T_KILSB-5 | ZENOBE KILMARNOCK SOUTH LTD | 51 | 59% | +0.39 |
| the other fifty six | ||||
| Oldham BESS | Field Oldham Ltd | 20 | 55% | +0.09 |
| Hawthorn Pit BESS | BP Gas Marketing Limited | 60 | 71% | +0.08 |
| Pillswood 2 Battery Storage | BP Gas Marketing Limited | 50 | 75% | +0.08 |
| Whitebirk BESS | Field Whitebirk Ltd | 25 | 81% | +0.04 |
| Newport BESS | Field Newport Ltd | 21 | 76% | +0.03 |
Whitebirk and Newport are moving four days in five, more than almost anything else on the list, and their position tracks the price barely at all. Burwell and the Kilmarnock South units move slightly less often and track it fifteen times more closely.
What it is not
The obvious explanation is that the unresponsive sites are selling reserve rather than trading, so their position is set by system need instead of by price. We tested that and it does not hold. Splitting the 66 into thirds by score, the share of money coming from reserve auctions runs 28 per cent, 33 per cent, 40 per cent. The direction is the wrong way round and the gap is far too small to explain a fifteenfold difference. Individual sites contradict it outright: one of the most price responsive sites on the list earns nothing from reserve at all, and one of the least earns 62 per cent of its money there.
It is not size. It is not how often they move, because that has been controlled for. It is not the trader, quite: sites under the same optimiser cluster loosely but not cleanly, and two Zenobe sites sit at opposite ends.
So we are telling you what we can see and asking
Something is driving a very large difference in how tightly these assets follow the price, and it is not visible in anything published. It could be duration, contract structure, a tolling agreement that removes the trading decision from the operator entirely, grid constraints at particular sites, or simply that some desks are better than others.
If you run one of these and you know which, we would like to know, and we will publish the correction. There is a methods section that exists for exactly this.
One more thing, which may be nothing
Great Britain's largest battery is Coalburn, five balancing units and about 512 megawatts. Over the same three weeks each of those units moved in between 6 and 11 per cent of half hours. It is entirely possible that is a commissioning programme rather than a business decision, and we say so rather than guess. But it is the biggest thing on the system and it was mostly still.
Twenty one days to 19 August 2026, 94 sites with enough half hours to score, 998 of 1,008 half hours carrying a price.