When four RTOs exceeded their summer peaks simultaneously before July even arrived, the Eastern Interconnection faced a scenario reliability planners had treated as theoretical. PJM, NYISO, ISO-NE, and MISO all strained against their limits during the same June heat wave—temperatures running 10 to 15 degrees above seasonal averages across a geographic footprint that normally allows stressed regions to import power from moderate ones.
That escape valve was closed. The entire interconnection was stressed at once.
NERC's 2024 State of Reliability Report had already documented the trend: 84 weather-related reliability events across the bulk power system, exceeding the 2019 baseline by 67%. The June 2025 event wasn't an outlier—it was the pattern accelerating.
The Physics of Failure
The mismatch between planning assumptions and operational reality has a straightforward cause: resource adequacy studies rely on 30-year historical weather datasets, but current climate conditions have already departed from that historical average. Extreme events occurring today exceed the frequencies implied by backward-looking data.
NERC acknowledged this gap in its 2024 Long-Term Reliability Assessment, recommending that planning studies incorporate forward-looking climate scenarios. But implementation lags recommendation. Most RTOs continue using historical weather in planning studies. The gap between what models assume and what operators experience widens each year.
The consequences compound during heat waves. Gas turbine output falls as ambient temperature rises—a unit rated at 200 MW may produce 170 MW when intake temperatures exceed 95°F. Steam plant efficiency drops when cooling water warms. Wind output collapses during high-pressure systems; during the June 2025 event, wind production across PJM fell to 15% of installed capacity. Even solar underperforms when panels overheat, with crystalline silicon efficiency dropping 0.4% per degree Celsius above 25°C.
Weather Events Now Drive Grid Emergencies
Weather Events Now Drive Grid Emergencies. Source: NERC 2024 State of Reliability Report. Heat events alone caused 40% of all weather-related grid incidents.
These derates are individually predictable. Collectively, they compound into systemic shortfalls precisely when demand peaks.
The Data Behind the Trend
NERC's reliability event data reveals clear clustering patterns. Extreme heat drove 34 incidents in 2024—high temperatures simultaneously cutting thermal plant output while spiking demand. Winter storms caused 23 incidents, with cold weather affecting gas supply, wind operations, and demand in parallel. Severe storms—hurricanes, derechos, tornadoes—caused 19 incidents through physical infrastructure damage. Drought conditions, primarily affecting the West, drove 8 events through reduced hydroelectric output and cooling water constraints.
The historical comparison is stark. Throughout Winter Storm Heather in January 2025, NERC reports that 90,500 MW of generation was unavailable, resulting in 5,400 MW of firm load shed. Winter Storm Elliott showed similar patterns, and the trend line points upward.
NERC's 2024 State of the Markets report projects summer peak demand growth at over 1.5% CAGR through 2034, with winter peaks growing even faster at nearly 2.0%. But this demand forecast lands against a transmission system that NERC's 2024-2025 assessment characterizes as "demonstrably constrained"—challenged by adverse weather conditions and transitions in resource mix simultaneously.
NERC's summer 2025 assessment forecasted net internal demand rising approximately 1.7%, or 13 GW, from 772 GW in summer 2024 to 785 GW in summer 2025. That demand increase arrived into a grid already showing strain.
Weather Events Up 67% Since 2019
Weather Events Up 67% Since 2019. Source: NERC 2024 Report. The June 2025 event wasn't an outlier—it was the pattern accelerating.
Global Parallels
The pattern isn't uniquely American. Australia's grid faces comparable challenges, with heat waves routinely triggering emergency conditions in South Australia and Victoria. The Australian Energy Market Operator now publishes heat wave reserve forecasts days in advance and implemented a Reliability and Emergency Reserve Trader mechanism that procures emergency reserves during forecast extreme conditions—effective buffer capacity, but at costs exceeding $1,000/MWh.
India experienced the consequences during its May 2025 heat wave, when peak demand in the Northern Region surged to 89 GW against installed capacity of 112 GW. Coal plant availability dropped to 71% due to fuel constraints. The grid survived through emergency protocols and import arrangements from neighboring regions—options that evaporate when heat waves span entire interconnections.
China's State Grid confronted its own version in 2024 across multiple provinces. Sichuan's drought cut hydroelectric output by 50%, forcing deployment of mobile generation and interprovincial emergency transfers. Industrial users faced load curtailments for two weeks—economic costs that flow directly to manufacturing competitiveness.
Regulatory Response Lags Events
FERC has opened Docket AD25-7 to examine extreme weather preparedness across RTOs, including planning methodology improvements and climate scenario analysis. NERC's cold weather standards (EOP-012-2) took effect in 2024, requiring generators to prepare for extreme cold—though compliance costs prompted generator opposition and implementation monitoring extends through 2025.
State proceedings move in parallel. California's SB 1020 mandates that the CEC incorporate climate projections into reliability planning, extending requirements beyond CAISO's existing process to distribution-level impacts. But regulatory proceedings operate on timelines measured in years while weather events accelerate on timelines measured in seasons.
Peak Demand Growth Outpacing Grid Capacity
Peak Demand Growth Outpacing Grid Capacity. Source: NERC 2024 State of Markets. Winter peaks growing faster than summer—challenging a grid designed for summer stress.
There is one bright spot in the reliability picture. NERC's assessment notes that batteries are demonstrably improving grid reliability during stress events—fast-responding resources that maintain output regardless of ambient temperature and can dispatch precisely when thermal plants derate. But battery deployments, while accelerating, remain a fraction of the capacity needed to offset weather-driven generation shortfalls across interconnection-wide events.
Strategic Implications
For generators, weather performance data increasingly determines capacity market standing. Units that underperform during extreme conditions face accreditation reductions that compound annually.
For load-serving entities, resource portfolios require diversification that accounts for weather-correlated performance degradation across technologies and geographies—portfolios designed assuming uncorrelated regional stress need fundamental reassessment.
For industrial customers, demand flexibility has become a reliability asset rather than merely a cost optimization tool. Interruptible service arrangements provide both savings and grid support, positioning flexible loads as de facto peaking resources.
For regulators, the central question is no longer whether planning assumptions require updating, but how quickly methodologies can incorporate climate projections despite inherent forecast uncertainties.
What Comes Next
Watch for FERC action on Docket AD25-7 through late 2025, where staff recommendations on climate scenario requirements could reshape RTO planning processes. NERC's upcoming 2025-2026 Winter Reliability Assessment will provide the first systematic evaluation of whether cold weather standard compliance has actually improved generator availability.
The fundamental physics won't change. Thermal plants will continue derating in heat. Wind will continue dropping during high-pressure systems. Demand will continue spiking during temperature extremes. The only variables are whether planning models catch up to operational reality—and whether infrastructure investments arrive before the next interconnection-wide stress event finds margins that aren't there.
About the Author
Dr. Sayonsom Chanda
Dr. Sayonsom Chanda is an electrical engineer and senior scientist with more than a decade of experience in developing AI, ML, and other advanced computing solutions for the electric utility industry in US and India. He is also an energy policy thinker and a published author with more than 20 papers and 1 book.
