The California Independent System Operator (CAISO) has published a report regarding electricity generation in the summer of 2025, which includes data through the end of September 2025. They provide a nifty interactive chart at that link, and the underlying data is easy to find in the source of the web page. I've included the data for July and September at the bottom of this post, as we're going to dive into those details in a moment.
Below is the graph of the sources of energy on the California electrical grid in September of 2025, the period ending about two weeks ago at the time of this writing. Importantly, the Solar resources shown are only utility connected solar arrays. Rooftop solar is behind the meter, reducing demand for electricity rather than adding to the supply measured here. There is approximately another 19 Gigawatts of solar capacity installed behind the meter.
This is the same data, shown as a percentage of the total. Because total electricity consumption varies throughout the day, the contribution of baseload sources like Methane-fired generators varies as a percentage of the total even though their output is constant.
A few observations:
- The contribution of wind power is smaller than I expected. I may get a skewed view of the prevalence of wind generation in California as I pass through the Altamont Pass wind farm regularly, but I expected wind to be a larger percentage.
- It is too small to be visible in the graph, but Solar power never actually drops to zero. It continues to supply about 4 Megawatts all night. I believe this might be Ivanpah, a solar thermal generation plant in the desert, which continues generating power from stored heat even after the sun has set.
I tried to incorporate behind-the-meter solar into a similar graph, below. This is a crude estimate:
- In 2024, 19 Gigawatts of rooftop solar was installed versus 21 GWatts of utility-scale. I made the assumption that the production from rooftop solar would be approximately 19/21 of the utility number.
- Prior studies show that rooftop solar is not installed in ideal locations nor properly angled toward the sun, I made the additional assumption that it would be 80% as productive as utility solar.
The result hews considerably closer to the 67% renewable result announced by the state last year.
Virtual Power Plants
I have combined the categories which CAISO reported separately as Batteries and Demand Response, because until 4/2024 Virtual Power Plants formed via aggregation of residential batteries like Powerwalls were contracted as Demand Response. Only utility-scale battery installations like Megapacks were accounted for as Batteries. Splitting into two categories obscures and substantially minimizes the true contribution of battery power to the grid.
We participate in a Virtual Power Plant in northern California, allocating about half of the 27 KWh of capacity installed at the house. For a number of days this summer in the late afternoon and early evening, the house supplied about 6 kilowatts of power back to the grid. It is quite smooth, we don't even notice unless we look at the app to see what it is doing.
Raw data from the CAISO report web page.
"September": [ [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1392.000662, 1319.840318, // Demand Response 1239.827827, 1177.653666, 1023.942241, 287.33, 0, 0], // Demand Response [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2264.012689, 6312.981683, // Battery Storage 10219.53932, 9740.80126, 7382.450552, 5789.963039, 2869.891625, 0], // Battery Storage [11665, 11665, 11665, 11665, 11665, 11665, 11665, 11665, 11665, 11665, 11665, // Imports 11665, 11665, 11665, 11665, 11665, 5500, 5500, 5500, 5500, 5500, 5500, 5500, // Imports 11665], // Imports [742.4458, 725.114696, 719.58188, 591.859728, 356.992952, 328.293048, 330.9584, // Wind 323.6145657, 277.992424, 205.762648, 221.590544, 199.863504, 216.38616, // Wind 311.644072, 316.5584821, 324.313968, 396.1921449, 535.622064, 785.470392, // Wind 905.120696, 1081.627632, 1164.064064, 1096.58392, 1058.725816], // Wind [3.7224, 3.7224, 3.95928, 4.43304, 5.02524, 7.1064, 155.30868, 3417.63696, // Solar 8622.78732, 11304.55656, 12390.19452, 12923.98668, 13035.8448, 12592.86717, // Solar 12166.76592, 11529.86232, 8779.161639, 5405.92308, 1148.5296, 12.79152, // Solar 5.34672, 4.01004, 3.8916, 3.84084], // Solar [1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, // Other renewables 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, // Other renewables 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, // Other renewables 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861, 1755.632861], // Other renewables [1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, // Other 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682, 1682], // Other [7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, // Hydro 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014, 7014], // Hydro [2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, // Nuclear 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280, 2280], // Nuclear [26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, // Methane 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, 26188, // Methane 26188, 26188], // Methane ... ]