California’s solar penetration has been front and center of conversation over the years as the local balancing regions continue to work towards meeting the Renewable Portfolio Standard requirements. Leading up to the month of May 2025, the narrative around the CAISO system needing to balance their midday block of hours by curtailing such resources held true, but how quickly things can change given the conditions on the grid, of which ties into the supply stack adjustments, weather conditions and the almighty battery fleet that is now present on a daily basis.

Figure 1 | CAISO Cumulative Curtailment Volume – Year on Year Comparison

The graph in Figure 1 displays the cumulative megawatt hours of renewable curtailments within CAISO for the past four years with the most recent assigned the bright blue colored line.  Looking at the progression from the middle of Q1 to the end of May 2025, the highlights are two fold with the first being how curtailments were in play earlier in the season compared to previous years and the second associated to the month of May when the area between 2025 and the previous year shrunk in a way that differs from the previous Q2 mid-month.  Unpacking the early start to curtailments starts with moderate weather conditions holding back any type of power demand for heating.  Throw in the fact that the California hydro season was taking on its third good precipitation pattern where the hydro storage facilities needed to release water to make room for the current snowpack accumulation.  Add to it, the health of the supply stack as both Diablo Canyon nuclear units were operating at full capacity and the simple fact that both the day-ahead and real-time CAISO operations were displaying stout S to N congestion results across the midday block of hours.