How we read what the fleet is doing right now
The two figures on the front page say what every battery in the balancing mechanism is doing at this minute, and what the whole fleet did over the last day. Both are measured. Neither involves a model. But there is one trap in the data that turns the answer upside down, and it is worth stating plainly.
The trap
Two separate things are published about each unit.
A physical notification is what the operator told the system it intends to do, submitted ahead of time, half hour by half hour.
An accepted bid or offer is what the control room has since told it to do instead.
An acceptance does not change the notification. The notification stays exactly where it was. So a battery that notified zero and was then accepted at plus thirty megawatts is discharging thirty megawatts while still saying, in the notification feed, that it is doing nothing.
Read notifications alone and you get the wrong answer, and you get it wrong at precisely the moments that matter, because that is when the control room is intervening. When we built this the first reading said 18 discharging and 18 charging. Applying acceptances over the top of it gave 2 discharging and 33 charging. Same minute, same data, opposite picture.
What we actually do
- Pull every physical notification for the 134 battery units on Elexon's balancing mechanism register, for the last 25 hours. 30 of them have not had a bid or offer accepted in the last year, so they have no revenue figure on this site, but they still publish a position and they are read here like any other.
- Pull every accepted bid and offer for the same units over the same period.
- Cut the day into five minute slots. Notifications are submitted per settlement period, so anything faster is theatre.
- In each slot, for each unit, take the accepted level if an acceptance is in force, and the notified level if not. Where two acceptances overlap, the later acceptance number wins, which is how a new instruction supersedes an old one.
- Add the units together for the fleet line. Report the last slot as the position now.
What it does not tell you
- It is not money. It is megawatts. The cash for the same period does not settle for another day and keeps moving for months.
- It is 134 units, not the whole fleet. It is every battery unit on the balancing mechanism register. Across Great Britain we can see 250 battery registrations that were paid in the last year, and those outside the balancing mechanism publish no position at all. So the true fleet swing is larger than the figure shown.
- It does not say which market a position came from. The notification is the net of everything the trader has already sold or bought for that half hour: the day ahead auction, intraday trading, forward deals, a toll. Wholesale arbitrage is inside the line, but the page cannot split it out or say what price was paid. What a battery earned outside the balancing mechanism is worked out separately, from metered output, on its own methods page.
- A level is an instruction, not a meter reading. It is what the unit was told to do or said it would do. Actual metered output is settled later and can differ. About two weeks later we lay the meter against the plan and show where it did.
- Sitting still is not the same as doing nothing. A battery holding at zero may be holding charge deliberately, or holding a reserve contract that pays it to sit there. Frequency response is delivered automatically, second by second, around the notified level, and appears in neither feed.
Tell us we are wrong
If you operate one of these units and our reading of your position is wrong, that is the most useful message we could get. Say which unit and which minute.