I-SEM Daily Briefing — 21 May 2026

Price Profile #

Std dev €14.76/MWh · Median €132.96/MWh · Periods above €150: 12 of 48 (25%)

A flat day. The flattest of the 19-day run, in fact.

Mean €136.98 is mid-range, but the volatility metrics all touched new lows: std dev €14.76, range €46.75, peak/off-peak spread €9.89. Every period sat between €119 and €166. No scarcity, no real trough.

The shape carries shape’s outline but none of its drama. Overnight ran €121–147 with wind at 52–62%. Morning ramp climbed €127 → €152 between 05:30 and 07:30 — present but modest. Midday belly held €119–148 from 09:00 to 15:30, with the floor at €119 at 15:00. Evening climb built from €126 at 16:00 to €166 at 20:00, then held — €163–166 across four consecutive hours from 19:00 to 21:30 before fading.

What’s notable is that plateau. Most evening peaks have a clear single-period top (May 20’s €184 at 21:00 was alone). Today’s evening peak runs at the same €165 level for eight half-hours. The merit order found its marginal plant for evening conditions and stayed there — no escalation, no scarcity, no peaker bids.

Price vs Wind #

Mean wind: 46.3% · Range: 32.2%–61.7%

Wind shape was the inverse of yesterday’s, and that single difference is the structural point of the day.

May 20: wind peaked overnight (62%), drained through the day, bottomed at 27% during the evening peak. Wind shortfall into the peak — produced a €184 high.

May 21: wind peaked overnight (62%), drained to its low (32%) in the afternoon (14:30), and then recovered through the evening — back to 46% by 20:00, 52% by 21:30. Wind refill into the peak — capped the high at €166.

Same demand (3,981 MW vs 3,985 MW yesterday), same mean wind level (46% vs 42%), opposite wind shapes, €19/MWh of peak difference. The takeaway is that wind level isn’t enough — the timing within the day is the bigger lever.

The teaching line: a 50%-wind day with the right shape clears at €165. A 50%-wind day with the wrong shape clears at €220+. The two days look identical in any daily summary statistic; they don’t look identical to a battery, a peaker, or a renewable generator.

Week in Context #

The wind-rich-working-day cluster continues to define the regime: May 11 (€120), May 13 (€110), May 14 (€127), May 19 (€120), May 20 (€128), May 21 (€137). Six days in the €110–137 band on similar wind/demand fundamentals, with the spread reflecting nothing but the intra-day wind shape.

Price Duration Curve #

Periods above €150: 12 (25% of day) · Above €200: 0 (0% of day)

12 above €150, 0 above €200 — same headline counts as yesterday. But the shape of the curve is different. Yesterday’s PDC had a clear top shoulder around €178–184 and a clear cheap trough at €100–110. Today’s PDC is a smooth, gentle slope from €166 at the top to €119 at the bottom, with no steps and no plateau. The flattest PDC of the run.

This is the “low-and-flat” archetype, minus the low. Wind has saturated the merit order enough that the marginal plant stays inside mid-merit gas across every demand condition the day produces — no excursion into peakers at the top, no surplus pushing the floor below gas-marginal at the bottom. A consumer’s PDC: predictable, uniform, dull.

Peak / Off-Peak Spread #

Peak avg (07:00–22:00): €140.69/MWh · Off-peak avg: €130.8/MWh · Spread: €9.89/MWh

Spread €9.89 — a new low for the run.

Peak avg €140.69, off-peak avg €130.80. The peak and off-peak windows averaged within €10/MWh of each other. There was effectively no time-of-use signal in the market today — the same plant set the price in the overnight hours as set it in the evening peak, just at slightly different load points.

A €9.89 spread sits below the round-trip efficiency threshold for storage (a 1MW/2MWh BESS needs ~€18 of spread to break even on a single cycle after 85% RTE), so on this metric alone the day is sub-marginal. The captured spread the optimiser actually found is wider (€43), because peak/off-peak averaging blurs the day’s real shape — but it’s still the worst captured spread of the run.

BESS Dispatch Signal #

Simulated 1MW/2MWh battery, one optimal DAM cycle. Gross before network charges and capacity costs.

€38 gross — the worst day of the 18-day run, by €18.

Charge €121 in the afternoon trough (14:00–15:30 at €121 avg), discharge €164 in the evening plateau (19:30–21:00 at €164 avg). Captured spread €43, gross €38 after 85% RTE eats into the discharge revenue. ROI 15.6% on the €242 charge cost.

A real BESS operator running RTE-aware optimisation would have a question to ask today: is the captured spread wide enough to justify the cycle at all? €43 of spread minus 15% of round-trip losses leaves perhaps €25/MWh of net spread before any wear-and-tear or opportunity cost. A discretionary operator might reasonably skip the day entirely — and that decision, applied across enough days, is what separates a model BESS portfolio from a real one.

Running BESS series: 18 days, cumulative €1,747, mean €97/day. Top 4 days (May 7, 15, 17, 18 = €624) now represent 36% of cumulative revenue from 22% of days. Today reinforces the concentration story rather than diluting it.

Commentary #

The flattest day of the 19-day run. Mean €136.98 is mid-pack, but every volatility metric (range, std dev, peak/off-peak spread) touched a new structural low. Every period sat €119–166. There was no scarcity tail and no cheap trough — just a uniformly priced day that the merit order solved at gas-marginal across all demand conditions.

The structural story is the wind shape. May 20 saw wind drain into the evening peak (62% → 27%) and produced a €184 high. May 21 saw wind recover into the evening peak (62% overnight → 32% afternoon → 52% by 21:30) and produced a €166 high. Same mean demand, similar mean wind, opposite intra-day wind timing — and €19/MWh of evening peak difference attributable to the timing alone. The bigger lever on price isn’t wind level; it’s wind shape. Today’s V-out-of-peak shape capped the merit order short of any escalation event.

For storage, the floor of the series. €38 gross, captured spread €43, ROI 15.6%. After 85% round-trip efficiency, the day barely justifies cycling. The 18-day cumulative now reads €1,747, with the four big scarcity days (May 7, 15, 17, 18) anchoring 36% of revenue from 22% of days. Give us a wind shortfall tomorrow.