Grid-scale batteries in CAISO earned $2.78/kW-month in May 2026. That is level with April's $2.77/kW, but $1.10/kW below May 2025's $3.88/kW, a 28.4% year-over-year decline.

The fleet has now held a $2-3/kW spring base for three straight months.

Average daily highs reached 73.9°F against 75.4°F a year earlier, and average lows held at 49.2°F versus 49.4°F. With temperatures essentially flat, price compression came from the supply side.

Solar generation fell 3.9% year-over-year while wind rose 33.1%. The increase in wind tracks SunZia, the 3.5GW New Mexico project that reached commercial operation in May 2026. This step up led to a surplus of supply in the evening that flattened the discharge window batteries depend on.

​The month also opened with the Extended Day-Ahead Market going live on May 1, bringing PacifiCorp's two balancing authority areas and roughly 12 GW of generation into CAISO’s day-ahead optimization.

Read last month's report here.

Daily TB4 spreads compressed across the entire month

Daily average TB4 fell from $165/MW to $119/MW year-over-year. The strongest day of May 2026 was May 11 at $174/MW, which would have ranked as an average day in May 2025. The weakest day, May 26, posted just $82/MW.