A couple of weeks ago our blog post touched on the surging renewable curtailments that have been present within CAISO over the past month. Solar expansion has continued at a strong rate so far in 2026, the hourly profile pushing higher in May to reach a new high for the midday with potential generation during hour ending 13 averaging 22.6 GW and increase of over 2 GW year-over-year. The situation has been further exacerbated by surging solar in the Desert Southwest and an improved water year for Pacific Northwest hydro that is leaving plenty of supply available at the interties in the middle of the day and no demand externally for CAISO’s excess solar generation. The hope that storage would be able to mitigate the worst of the midday oversupply issues has dimmed, at least for the time being, as CAISO batteries have not been up to the task despite increasing their activity in the energy markets each month of 2026. CAISO has responded by rapidly ramping up renewable curtailments during May, pushing even higher since the our previous blog. CAISO curtailments and the renewable situation in the West is covered in detail in our latest Monthly Renewable Report.

Figure 1 | CAISO Curtailments with Real-Time LMP

Back in mid-May CAISO was making waves as solar curtailments reached as high as 8 GW for several days. Over the weekend the issue grew even more pronounced with total solar curtailments closing in on the 10 GW mark on the 30^th of May, as shown in the figure below. Year-to-date solar curtailments skyrocketed during May, jumping from even with 2025 levels with around 2.3 TWh at the beginning of May to a whopping 3.7 TWh at the end of the month, already surpassing the 2025 end-of-year total by 5%.

Figure 2 | CAISO Renewable Curtailments, Running Total by Year

It is not just batteries that are being pushed to help deal with the midday oversupply issues in CAISO. CAISO’s hydro resources are also being flexed far more than in years past to attempt to reduce production as much as possible during peak solar hours, only to ramp up hard in the evening as the solar output disappears. The figure below plots average hourly CAISO hydro generation by month going back to 2022. April and May’s profiles show the extent to which hydro is being stretched; CAISO hydro averaged under 500 MW of output for hour ending 14 in both months, far lower than at any point in the preceding five years only to surge up to 4.4 GW of generation on average for hour ending 21 in May.

Figure 3 | CAISO Hourly Average Hydro Generation (MW)

CAISO will continue to lean on its resources as much as it can but the oversupply issues stemming from solar growth in California and the Desert Southwest are impossible to fully mitigate.

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