​Great Britain's new Slow Reserve product cleared its first auction on 31 March 2026. Gas took 75% of the 1,800 MW requirement. Batteries showed up overnight and disappeared by morning. The auction's welfare-maximising algorithm cleared two periods at £0/MW/h despite no unit bidding at zero. And in the Balancing Mechanism, holding a Slow Reserve contract made no difference to how much batteries were used, despite technical requirements implying longer dispatches.

Gas dominated day one - as expected

NESO procured 1,800 MW of Positive Slow Reserve per settlement period across its first 24-hour window. Gas reciprocating engines cleared around 630 MW in most periods; roughly a third of total volume. OCGTs added another 350 to 420 MW.

Batteries peaked at 313 MW overnight when prices dropped below £3/MW/h, then shrank to 21 MW by the morning peak. No surprise, as Slow Reserve limits battery flexibility to operate in other markets. The only battery units that did pick up contracts were two-hour units, as they have an easier time managing state-of-charge than one-hour systems.

Clearing prices that no-one bid at - how does this happen?

The EAC auction uses a welfare-maximising algorithm. It does not simply pay the marginal bid. It maximises total welfare, redistributing value from abundant periods to scarce ones. At 03:00 and 03:30, supply comfortably exceeded 1,800 MW. The algorithm pushed the clearing price to £0 and shifted the surplus welfare into tighter periods like the early evening, when prices hit £15.86/MW/h. Without welfare sharing, prices would have been even higher during these periods

The accepted bids follow the expected shape: higher in morning and evening peaks, lower overnight. The clearing price does not. It sits flat at £6.50 for nine consecutive periods between 09:30 and 13:30, then locks at £12.94 for twelve periods from 14:00 to 19:30. That is welfare redistribution in action — smoothing surplus across the day rather than paying the marginal bid in each period. This means NESO can clear the auction at the lowest possible cost, while operators still receive the revenues they have bid for.

Slow Reserve prices are likely to fall

Indian Queens OCGT bid at 1p every single settlement period throughout the first day. It was accepted in every period and will earn almost £240/MW across the day - not from its own bid price, but from the clearing price set by other units.

If more peaking plants followed suit and bid as price-takers, the supply curve would shift downward and clearing prices would compress. Quick Reserve offers a precedent. Battery prices in Quick Reserve were high in the first month but started to reduce afterwards as participants learned the market and competition increased.

Slow reserve made no difference to Balancing Mechanism dispatch length

Batteries that hold a Slow Reserve contract must be able to dispatch continuously for half an hour. This brings in the question: should operators get a short-term reserve contract to get longer dispatches in the balancing mechanism?

On day one, Slow Reserve contracts had no impact on Balancing Mechanism usage.

Across 16 Slow Reserve battery units with 464 dispatches, the mean duration of Balancing Mechanism Offers was 7 minutes. For 43 batteries that performed Quick Reserve on the same day, it was 8 minutes across 1,272 dispatches. The distributions were nearly identical — 77% of dispatches in both groups lasted between 2 and 10 minutes.

Day one was a calm system day. During tighter margins, when NESO needs to hold reserve for extended periods, we may see Slow Reserve units being dispatched for longer. For now, a Slow Reserve contract is not impacting battery utilisation in the Balancing Mechanism.