The relentless grip of a June 2025 heatwave across the Mid-Atlantic tested the resilience of PJM Interconnection, the largest grid operator in North America. As temperatures soared into the 90s and even triple digits in parts of New Jersey, PJM issued a series of critical alerts to ensure the lights stayed on, highlighting the increasing strain on an aging grid facing surging demand. These pre-emptive measures, while common in extreme weather, underscore a growing concern about the delicate balance between electricity supply and demand, a balance that, if lost, could lead to severe consequences.
PJM’s Alerts in the June 2025 Heatwave
Throughout June 2025, PJM escalated its operational alerts as a persistent “heat dome” blanketed its vast service territory. New Jersey, in particular, experienced a grueling stretch of heat, with temperatures consistently in the mid to high 90s, and heat indices pushing well into triple digits. This intense heat directly translates to skyrocketing electricity demand, primarily driven by air conditioning usage in homes and businesses.
PJM’s initial response included Hot Weather Alerts, a routine procedure issued ahead of forecasted 90-plus-degree weather. These alerts prepare transmission and generation personnel for increased demand and signal a recall of scheduled generator maintenance to ensure maximum available capacity.
As the heat intensified and demand forecasts climbed, PJM issued Maximum Generation Alerts and Load Management Alerts, often concurrently, for multiple days, extending them through late June. For instance, on June 23, PJM forecasted a system-wide load of approximately 161,000 MW, significantly exceeding its initial summer peak forecast of 154,000 MW (PJM Inside Lines, 2025a). These alerts, which were also accompanied by a NERC Energy Emergency Alert Level 1 (EEA-1), are directed at transmission and generation owners, urging them to defer any maintenance or testing and ensure all available units operate at maximum output. The Load Management Alert serves as a pre-notification that demand response resources may be called upon. Critically, these alerts do not typically require direct action from individual customers, but rather prepare the system for potential stressors (PJM Interconnection, n.d.-a).
Not the First Time
PJM’s use of Maximum Generation and Load Management Alerts during summer heatwaves is not new. The grid operator has a long history of employing these tools as part of its emergency procedures to maintain reliability during periods of high stress. Similar alerts have been issued in previous hot summers, such as June 2024, when a heatwave also prompted warnings and saw demand rise significantly across the Eastern Interconnection (U.S. Energy Information Administration, 2024).
These alerts demonstrate PJM’s proactive approach to grid management, designed to be steps in a ladder of emergency actions, allowing grid operators to prepare resources and signal potential issues before they become critical. However, the increasing frequency and intensity of these alerts in recent years, particularly in 2025, reflect a broader trend: accelerating electricity demand (driven by data centers and electrification of the home and transportation sectors) coupled with the retirement of older power plants, leading to tighter reserve margins (Enel North America, 2025; Enverus, 2025). This shrinking cushion means that PJM is operating with less flexibility than in previous decades, making each heatwave a more significant test of the grid’s resilience.
What Happens If Load Exceeds Supply?
If, despite all pre-emptive measures and alerts, electricity load genuinely exceeds supply, a series of increasingly severe events would unfold, potentially leading to widespread power outages. The primary consequence is a drop in the grid’s frequency. The electrical grid must maintain a precise frequency (60 Hz in North America). When demand outstrips generation, the generators effectively slow down, causing the frequency to decline.
Grid operators like PJM have a multi-tiered response:
- Automatic Responses: Generators have governors that automatically increase output in response to frequency drops. Synchronized reserves (online generators operating below full capacity) are quickly ramped up.
- Operator Actions: If frequency continues to drop, PJM would escalate emergency procedures, moving to a NERC EEA Level 2 (EEA-2). This might involve:
- Voltage Reduction: Slightly lowering the voltage on the distribution system, which subtly reduces consumption without noticeable impact to most users.12
- Public Appeals for Conservation: Urging customers to voluntarily reduce non-essential electricity use.
- Emergency Demand Response: Activating contracted demand response programs, where large industrial or commercial users are paid to temporarily reduce their electricity consumption (PJM Interconnection, n.d.-b).
- Manual Load Dump Action (Load Shedding): This is the last resort to prevent a catastrophic, uncontrolled blackout. If all other measures fail, PJM would direct local electric distribution companies (like PSE&G, JCP&L, Atlantic City Electric in New Jersey) to implement rotating customer outages (also known as rolling blackouts). These are typically short, controlled power cuts (e.g., 30-60 minutes) to specific areas, designed to reduce demand rapidly and bring the grid back into balance (PJM Manual 13, 2025). This prevents an uncontrolled collapse of the entire system.
- Blackout/Grid Collapse: If manual load shedding is not implemented quickly or effectively enough, or if there’s a sudden, severe, and widespread loss of generation, the grid frequency can fall too far. This causes power plants to automatically trip offline to protect their equipment, leading to a cascading failure and a large-scale, uncontrolled blackout. Restarting a grid after such an event is a complex and lengthy process.
While PJM regularly faces tight operating conditions and has called for load curtailment from demand response participants, a full, uncontrolled system-wide load exceeding supply resulting in widespread forced outages across its entire footprint, similar to a “blackout” scenario, has been successfully avoided through its robust emergency procedures. PJM’s design aims to prevent this exact scenario through its multi-layered reserve requirements and emergency operating procedures, including the ability to shed load proportionally if absolutely necessary.
Historically, PJM has successfully managed to prevent widespread, uncontrolled blackouts, unlike some other grids that have experienced such events (e.g., Texas’ ERCOT grid during Winter Storm Uri in 2021). PJM’s effectiveness is a testament to its operational protocols and market mechanisms, even as the challenges of resource adequacy continue to grow. While there have been localized outages due to equipment failures or severe weather, PJM has consistently maintained the overall integrity of its bulk power system, preventing a full system collapse when faced with extreme load conditions.
The June 2025 heatwave served as a stark reminder of the challenges PJM faces in balancing increasing demand with an evolving generation fleet. The alerts issued were a critical part of PJM’s strategy to maintain reliability, demonstrating the continuous vigilance required to keep the lights on for millions of customers.
SOURCES
Enel North America. (2025, June 30). Record demand, real results: Demand response delivers in June 2025. Retrieved from https://www.enelnorthamerica.com/insights/blogs/demand-response-june-2025-heatwave
Enverus. (2025, July 3). It’s getting hot in here | Heat Dome in PJM. Retrieved from https://www.enverus.com/blog/its-getting-hot-in-here-heat-dome-in-pjm/
PJM Inside Lines. (2025a, June 19). June 23 Update: Maximum Generation Alert Issued for June 24. Retrieved from https://insidelines.pjm.com/pjm-issues-hot-weather-alert-for-expected-heat-wave-june-22-25/
PJM Inside Lines. (2025b, June 24). June 24 Update: Maximum Generation Alert Extended to June 25. Retrieved from https://insidelines.pjm.com/june-24-update-maximum-generation-alert-extended-to-june-25/
PJM Interconnection. (n.d.-a). Emergency Procedures: Message Definitions. Retrieved from https://emergencyprocedures.pjm.com/ep/pages/messagedefinitions.jsf
PJM Interconnection. (n.d.-b). Demand Response. Retrieved from https://www.pjm.com/-/media/DotCom/about-pjm/newsroom/fact-sheets/demand-response-fact-sheet.pdf
PJM Interconnection. (2025, February 20). PJM Manual 13: Emergency Operations (Revision 95). Retrieved from https://www.pjm.com/-/media/DotCom/documents/manuals/m13.pdf
PJM Interconnection. (2025, May 9). PJM Summer Outlook 2025: Adequate Resources Available for Summer Amid Growing Risk. Retrieved from https://www.prnewswire.com/news-releases/pjm-summer-outlook-2025-adequate-resources-available-for-summer-amid-growing-risk-302451333.html
PJM Interconnection. (2025, January 30). 2025 Long-Term Load Forecast Report Predicts Significant Increase in Electricity Demand. Retrieved from https://insidelines.pjm.com/2025-long-term-load-forecast-report-predicts-significant-increase-in-electricity-demand/
U.S. Energy Information Administration. (2024, July 1). June heat wave increased electricity demand in the eastern and midwestern United States. Retrieved from https://www.eia.gov/todayinenergy/detail.php?id=62410
Weather New Jersey. (n.d.). Weather New Jersey in June 2025: Temperature & Climate. Retrieved from https://en.climate-data.org/north-america/united-states-of-america/new-jersey-1058/r/june-6/
