California's Batteries Are Reshaping the Solar Day
Midday charging and evening discharge grew sharply in three years. CAISO's operating data shows how storage supports the solar day—and why the opportunity changes by location.
California's solar expansion is changing the work its batteries do after sunset. On the average May day in 2026, CAISO's battery series showed charging of about 7.3 GW near noon and discharge of 8.9 GW around 8 p.m., compared with 1.6 GW and 2.3 GW three years earlier. Storage has become a much larger part of the transition from the solar afternoon to the evening.
The operating pattern accompanies a state-level milestone: solar supplied 51 percent of electricity generated inside California in May, according to preliminary EIA data compiled by Ember. Ask the Grid's analysis of CAISO operating data shows how the growing storage response fits around solar production, and why the value of that response depends on where electricity can move.
Solar growth is part of a regional supply balance#
The state's generation share includes estimated small-scale solar as well as utility-scale plants. It measures electricity produced inside California, while imports also serve California consumption. CAISO's operating feeds cover a different system boundary: the ISO footprint and its utility-scale resources, alongside municipal balancing areas elsewhere in the state.
Imports were becoming more important in that balance. EIA found that, through the first five months of 2026, CAISO demand rose 7 percent from the same period in 2024 while in-area net generation fell 19 percent and imports doubled. New SunZia wind from New Mexico and a wetter Pacific Northwest supplied part of the difference. Solar's larger share of in-state generation therefore coincided with a greater contribution from neighboring systems.
Inside CAISO, the daily timing of solar and storage reveals another part of the adjustment. Batteries charge heavily through the solar window and supply power later, when solar output has declined and evening demand remains.
Batteries became the second half of the solar day#
In May 2023, the average battery profile reached about 1.6 GW of charging near noon and 2.3 GW of discharge after sunset. By May 2026 those levels had risen to 7.3 GW and 8.9 GW. The change is visible in both the depth of midday charging and the size of the evening response.
Across May, the aggregate series recorded 1.639 TWh during net-charging intervals and 1.416 TWh during net-discharging intervals. Eighty-six percent of net charging fell between 9 a.m. and 5 p.m.; 72 percent of net discharge fell between 5 p.m. and 11 p.m. The timing is consistent with storage shifting supply from the solar window into the evening. These totals describe net fleet activity, with simultaneous charging and discharging offset in the aggregate.
May net charging grew from 306 GWh in 2023 to 1,639 GWh in 2026, a 5.4-fold increase. Relative to utility-scale solar production in the same feed, it was equivalent to 33.4 percent of the monthly total, up from 8.2 percent. That scale comparison shows how large storage activity has become alongside solar output; the timing of discharge explains its operating role.
Two output highs carried different market signals#
The largest battery contribution relative to load arrived on March 29: five-minute output of 12.2 GW was equivalent to 43.8 percent of the containing hour's average CAISO-TAC load, 27.8 GW. This is a proxy for battery share using two time resolutions. The highest absolute output in the February 2023–July 29, 2026 series came later, on July 9, at 13.0 GW, equivalent to 36.6 percent of that hour's 35.5 GW average load.
March's smaller output high coincided with much wider differences in electricity prices. The average of the NP15, SP15 and ZP26 hubs moved from −$6.60/MWh around midday to $50.10/MWh in the evening, peaking near $120/MWh. In the 6:45 p.m. real-time pricing interval, the middle 90 percent of 18,748 CAISO-market nodes ranged from −$151/MWh to $158/MWh. Eighteen percent remained negative while 29 percent exceeded $100/MWh.
July's 13 GW output high arrived in a more uniform market. In the 15-minute RTPD interval beginning 8:15 p.m., containing the 8:20 p.m. output observation, the median across 19,316 priced nodes was $42.20/MWh. The middle 90 percent lay between $39.50/MWh and $44.30/MWh, with every node below $100/MWh. The contrast puts location and timing alongside the size of the storage response.
Location determines the opportunity#
For a battery owner, March's price dispersion makes charging access and evening deliverability central to the operating strategy. A site at a node still priced below zero faced a different opportunity from one above $150/MWh, even while both were part of the same regional storage response.
The comparison also matters to system operators. A large aggregate discharge can occur while market prices remain relatively uniform; a smaller response can coincide with large differences between locations. Tracking the system's battery curve together with congestion and local resource availability gives a fuller picture of the evening ramp.
For planners, solar production, imports and storage form a connected operating balance. The monthly generation share describes the changing supply mix; net charging and discharge describe when storage acts; nodal prices show how differently that transition can be valued across the market.
As storage output grows, the next spring surplus and evening ramp will test whether the March pattern recurs: substantial regional battery discharge alongside persistent local price separation. Repeated intervals of that kind would make charging access and transmission delivery an increasingly important part of the next storage investment.
The analysis is frozen at 12:30 p.m. Pacific on July 29, 2026. Sources are CAISO's renewable reports, five-minute net storage supply, hourly CAISO-TAC load and 15-minute RTPD prices. The storage series includes standalone and hybrid resources and covers February 1, 2023 through the cutoff, with 99.93 percent of expected intervals observed. Net energy integrates the published intervals. Battery-share proxies compare five-minute output with the containing hour's average load. Nodal distributions cover the published CAISO-operated real-time market, using the latest revision of each node and interval; RTPD timestamps mark interval starts. Explore CAISO on the map.
