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CURTAILMENT TRACKER: Cal-ISO wind, solar curtailments in August higher year on year


Localized curtailments up 10.5 points year on year

CAISO renewable capacity up 7.6% year on year

Curtailments increased 7.8% month on month

California Independent System Operator wind and solar generation curtailments climbed 58.3% year on year in August as installed renewable capacity has increased 1.762 GW from year ago levels while demand decreased year on year.

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ISO curtailments rose 7.8% month on month to 36.736 GWh in August as ISO peakload decreased 1.2% from July and was down 4.8% year on year to average 37.55 GW, according to ISO data. Curtailments reached an all-time record of 341.959 GWh curtailments in March.

Localized curtailments accounted for about 84.5% of the renewable generation curtailed in August and have made up 78.8% of curtailments from January through August, according to ISO data. In comparison, localized curtailments accounted for 68.3% of the 1,285,193 MWh curtailed from January through August 2020, according to ISO data.

There are two types of curtailments. Systemwide curtailments occur when there is an oversupply, and localized curtailments occur when the amount of power from one transmission point to another point is controlled for congestion management.

Generation changes

Imported generation was down 5% year on year to average 150.349 GWh/d in August, as hydro generation output sank 37.3% year on year to average 36.428 GWh/d, according to ISO data. Solar generation output increased 13% to average 114.019 GWh/d in August as wind generation output was little changed at around 51.850 GWh/d.

Total generation shed 4.4% year on year to average 30.267 GWh/d in August.

Thermal generation output slipped 5% year on year to average 285.993 GWh/d in August as SoCal city-gate spot gas prices were up 50% year on year to average $6.115/MMBtu for the month, according to S&P Global Platts pricing data.

Wholesale power prices were split. SP15 on-peak day-ahead locational marginal prices dropped 35.8% year on year to average $63.11/MWh in August, while NP15 on-peak day-ahead LMP rose 8.1% year on year to average $63.78/MWh, according to ISO data.

CAISO installed renewable capacity increased 7.6% year on year in August to 24.877 GW with solar accounting for 57.8% and wind making up 28.5%, according to the ISO's August Key Statistics report. The remaining 13.8% comes from geothermal, small hydro and biofuels.


So far this month, wind and solar curtailments have totaled 13.466 GWh through Sept. 20, 49.3% lower than the same period in September 2020, according to ISO data.

SP15 on-peak balance-of-the-month was valued in the upper $70s/MWh on the Intercontinental Exchange Sept. 20, 13% below where the daily traded. The SP15 on-peak September package averaged $94.07/MWh in its last month, 83% higher than the 2020 package average a year ago due to summer supply concerns with a weak hydro outlook, continued drought and above-normal temperatures, according to S&P Global Platts data. On-peak October is currently in the low $90s/MWh with November in the mid-$90s/MWh, 147% and 138% higher than their 2020 counterparts a year ago.

There is a greater probability for above-normal temperatures across California in the six- to 10-day forecast as well as the three-month temperature forecast, according to the US National Weather Service.