California’s electricity grid faces challenges during periods of high demand on hot days primarily due to a combination of surging air conditioning loads, the timing mismatch with solar generation (the “duck curve”), limited flexible dispatchable resources during evening ramps, transmission constraints, and compounding risks like wildfires.
Peak Demand Dynamics on Hot Days
- High Temperatures Drive AC Usage: On extreme heat days, electricity demand spikes significantly, often in the late afternoon and early evening (typically 4–9 p.m.), as residents and businesses run air conditioners heavily. Peak loads can approach or exceed 46,000–52,000+ MW during severe events.
- Evening Ramp Challenge: This peak often coincides with or follows the rapid drop-off in solar generation at sunset, creating a steep “neck” in the net load curve that requires fast-ramping resources.
The Duck Curve and Renewable Integration
California’s heavy reliance on solar power creates a pronounced duck curve:
- Midday: Abundant solar reduces net load (sometimes to very low or negative levels), leading to potential overgeneration.
- Evening: As solar fades, net demand surges sharply while AC loads remain high. This requires rapid ramp-up of other resources (batteries, gas plants, imports).
Batteries (now over 10,000–17,000 MW in recent years) have helped significantly by storing midday solar and discharging in the evening, reducing the need for Flex Alerts in recent summers.
Other Key Contributing Factors
- West-Wide Heat Events: When heat affects the entire Western U.S., neighboring states have less surplus power to export to California, reducing import availability.
- Resource Availability and Outages: Thermal plants (gas) can face reduced efficiency or outages in extreme heat. Hydro power may be limited by drought. Wildfires can damage transmission lines or force preemptive shutoffs (PSPS).
- Transmission and Distribution Constraints: Aging infrastructure, geography (mountains, earthquakes), and permitting delays slow upgrades. Localized distribution issues can cause outages even if the bulk grid is stable.
- Rising Overall Demand: Electrification (EVs, buildings, data centers) is increasing baseline and peak loads, though California’s per-capita usage remains relatively low due to efficiency and climate.
Historical Context and Improvements
In 2020, extreme heat led to rolling blackouts due to a combination of record demand, declining solar, insufficient reserves, and some generation shortfalls. Since then, the state has added tens of thousands of MW of new resources (especially batteries and solar), improved planning, and built contingency reserves. As of 2026 assessments, the grid shows surpluses under normal conditions but remains vulnerable to prolonged West-wide heat plus wildfires or other disruptions.
In summary, the core issue on hot days is not overall annual energy shortage but instantaneous supply-demand balance during the critical evening ramp, exacerbated by weather, renewable timing, and infrastructure limitations in a high-ambition clean energy transition. Batteries, demand response, and imports help, but overlapping extreme events can still strain the system. For real-time conditions, check CAISO’s Today’s Outlook.