Negative 8 PM capacity margin puts evening on imports and reserves

The decision window runs Aug 22, 5 PM to 9 PM CDT. The calculated capacity margin falls to -2,603 MW at 8 PM, the 4th percentile of its trailing 90-day range, even as the forecast load peak eased 487 MW.

Three developments lead: the bottom-percentile margin itself; North Hub's local residual at -$3.9/MWh, a -$1.1/MWh change, against South Hub's $4.2/MWh, a -$0.8/MWh change; and MetisCast's P50 path pricing $17.4/MWh below the cleared day-ahead curve across 17 hours, with a $62.6/MWh gap at 8 PM CDT.

Watch the cross-location median RT-DA spread against its -$9.7 to $8.5/MWh band, realized load against its 2,097 MW tolerance, and North Hub's residual for further separation.

Yesterday's day-ahead market cleared a substantial evening premium at every hub: North Hub's award peaked at $136.8/MWh, West Hub at $136.1/MWh, Houston Hub at $118.1/MWh and South Hub at $103.3/MWh, all at the Aug 21, 8 PM CDT hour. Real time settled far below those awards a full hour earlier, at the Aug 21, 7 PM CDT hour: North Hub topped out at $50.5/MWh, West Hub at $50.6/MWh, Houston Hub at $49.2/MWh and South Hub at $48.3/MWh.

That gap produced a systemwide RT-DA convergence spread of -$13.2/MWh over the trailing 24 joined hours, the softest reading in its trailing 90-day distribution. North Hub carried the widest raw spread at -$17.0/MWh, followed by West Hub at -$14.8/MWh and Houston Hub at -$11.9/MWh, with South Hub narrowest at -$8.9/MWh. The systemwide convergence miss explains most of each hub's raw number; North Hub's own local residual has widened beyond its calibrated band, a node-level shift covered separately below.

Against this, the prior issue's day-ahead curve missed the realized level outright even as it called the shape correctly: every selected hub's price peak landed together, just a full hour earlier than the curve implied, and well under the levels awarded.

Today's cleared day-ahead curve already prices a smaller evening: ERCOT Hub Average averages $44.6/MWh with a peak of $101.5/MWh at Aug 22, 8 PM CDT, down from yesterday's award, which averaged $42.3/MWh and peaked at $123.6/MWh. ERCOT's Operating Condition Notice spanning Aug 20 through Aug 25 flags peak demand nearing the system's all-time high on four separate days in that window, context for tonight's ramp, not an explanation for the RT-DA miss, which this data does not identify. Watch whether tonight's convergence spread holds inside its recent range or breaks lower, repeating the curve's pattern of overstating the evening. Blog | Heat, What Heat?

MetisCast prices ERCOT Hub Average -$17.4/MWh against the cleared day-ahead curve across 17 joined hours. Its P50 tops out at $43.4/MWh at Aug 22, 7 PM CDT while the market cleared $101.5/MWh at Aug 22, 8 PM CDT. The two agree on when the evening matters and disagree on what it pays.

Over the trailing 24 hours ending at the Aug 22, 8 AM CDT cutoff, the RT-DA convergence spread splits into a shared systemwide component of -$13.2/MWh plus a hub-specific residual. North Hub's mean spread of -$17.0/MWh is mostly that common miss, but its residual widened to -$3.9/MWh, moving outside North Hub's own calibrated band. South Hub's mean spread of -$8.9/MWh, the narrowest of the four hubs, carries a residual of $4.2/MWh that also breaks its band, the widest local separation of the group.

West Hub (-$14.8/MWh) and Houston Hub (-$11.9/MWh) sit inside their own residual bands, still part of the systemwide miss rather than a separate move. North and South pulling apart while the system spread holds its band is the pairing to track into tonight's 5 PM to 9 PM CDT window.

ERCOT's Operating Condition Notice for Aug 20 through Aug 25 flags peak demand clearing an all-time high on four separate days in that stretch, the backdrop for tonight's ramp. Against that backdrop, forecast generation outage capacity for the operating day sits at 7,704 MW, the 9th percentile of its trailing 90-day same-hour distribution, an easing factor working against tonight's negative capacity margin. Solar carries the midday hours and wind's forecast high lands before dawn; once solar falls off after sunset, thermal generation sets price through the evening ramp. Blog | Heat, What Heat?

MetisCast's leakage-safe P50 path for ERCOT Hub Average prices that ramp well under the cleared curve: the model calls $43.4/MWh at Aug 22, 7 PM CDT while the cleared day-ahead award peaks at $101.5/MWh an hour later, at Aug 22, 8 PM CDT. The gap averages -$17.4/MWh across 17 joined hours and widens to $62.6/MWh in that 8 PM hour, the model's largest single-hour divergence; its own uncertainty band is widest earlier in the ramp, reaching $43.7/MWh at Aug 22, 7 PM CDT, so the call carries real dispersion rather than a tight consensus read, worth tracking as the ramp unfolds tonight.

Tonight's decision window runs Aug 22, 5 PM CDT to 9 PM CDT, bounded by the evening's load and price peaks. North Hub and South Hub carry the two residuals now outside their trailing-90-day bands; keep intraday hedges live through the window rather than assume the cleared day-ahead premium fully settles the exposure. West Hub and Houston Hub sit inside their own bands, at the 69th and 68th percentile of local-residual history, so no separation case applies there yet. Two triggers frame the call: the cross-location median RT-DA spread staying between -$9.7/MWh and $8.5/MWh keeps the system-wide read inside its calibrated error, and load holding within 2,097 MW of the frozen forecast keeps demand out of the attribution question. A break on either side sends the desk to the hub detail grid and the market map to find which node moved.

Forecast-vintage policy. Every input in this issue was published at or before the knowledge cutoff. Forecast series are the latest vintage available at that instant and are never revised after the fact; realized history used for percentiles is guarded by the same cutoff. The preceding issue's frozen vintage is stored with the issue, so a revision is measured against what was actually published, not against a later correction. Not covered by this issue's data: Residual dispatchable demand; Reserve, PRC and contingency coverage; Weather-to-market translation. Each limitation is stated where it applies rather than filled in by inference.

Source and quality note. 29 of 29 requested issue series were available; 16 longer-lookback series provided the historical baselines. 37 event records were known at cutoff and 2 unchanged records were suppressed. Knowledge cutoff: Aug 22, 8:00 AM CDT.