A heat wave that settled over the eastern United States in the last week of June pushed electricity demand in the PJM Interconnection to levels not seen in 14 years. PJM, which operates the grid and wholesale market across 13 states and the District of Columbia and serves about 67 million people, reported preliminary hourly peak loads of 161,770 MW on June 23 and 162,401 MW on June 24, both between 5 p.m. and 6 p.m. Those are the third- and fourth-highest peak loads in the operator's history.
PJM's all-time summer peak remains 165,563 MW, set in 2006. The June 24 figure was nearly 10,000 MW higher than PJM's 2024 peak and about 15,000 MW higher than the 2023 peak, according to the grid operator.
Prices and fuel mix
The heat drove sharp moves in wholesale prices. The US Energy Information Administration reported that real-time prices in PJM peaked at $1,334 per megawatt-hour at 7 p.m. on June 23, compared with peak prices of $52/MWh a week earlier. EIA's Hourly Electric Grid Monitor recorded a peak of 160,560 MW on June 23, above PJM's seasonal peak forecast of 154,000 MW.
At the June 23 peak, EIA data show that natural gas supplied 44% of PJM's generation, nuclear 20%, coal 19% and solar 6%, with the rest from hydro, wind, oil and other sources. Oil-fired generation, typically the most expensive and used only at very high demand, was three times higher than in the same hour a day earlier.
The heat moved east the next day. ISO New England reported peak demand of 25,898 MW on June 24, the region's highest since 2013, with real-time prices peaking at $1,110/MWh. At that peak, 47% of New England's generation came from natural gas, 12% from oil and 12% from imports, mainly from Canada, according to EIA.
How PJM managed
PJM issued a Hot Weather Alert on June 19 covering June 22 through June 26, and required generation and transmission owners to defer scheduled maintenance and return units to service. It then issued a Maximum Generation Alert for June 23 through June 25, which tells generators to stay online and ready for dispatch, warns neighboring regions that PJM exports may be cut, and signals that demand response may be used.
PJM deployed demand response in select zones on June 23 and June 25 and across its whole footprint on June 24. The operator says this reduced load by more than 4,000 MW during the most extreme hours. Demand response consists of customers, mostly commercial and industrial, who are paid to reduce consumption when called.
Generator outages ran about 5 GW higher than PJM had assumed in its summer outlook, according to the operator. PJM still maintained its reserve requirements at all times. Load forecasting was accurate, with very low errors on the hottest days.
Heat stress levels
PJM's executive director of system operations noted that the temperature-humidity index, which combines temperature and humidity to measure heat stress, reached 84 across the footprint on multiple days and stayed there for hours at a time. PJM has recorded a value of 85 only twice in its history. Temperatures reached 95 degrees Fahrenheit or higher across much of the region on June 24.
Humidity matters for electricity demand because it affects how hard air conditioners work and how long they run. A hot and humid day produces higher sustained load than a hot and dry one.
Weather and data centers together
PJM attributes the higher peaks to a combination of extreme heat and growing demand from data centers. In its account of the heat wave, PJM said supply and demand conditions are tightening for two main reasons: power plants are retiring, largely because of state and federal decarbonization policies and economic pressures, while demand is rising, largely because of data centers. PJM's 2025 long-term load forecast projects summer peak demand climbing to about 220,000 MW within 15 years.
Data center load is relatively flat across the day and the year, so it adds to the baseline on which weather-driven demand sits. On a hot afternoon, air conditioning load rises on top of an already higher base. That combination is why peaks in 2025 exceeded those in 2023 and 2024 by such wide margins, even though the 2006 record was not broken.
Implications for capacity markets
The heat wave came weeks before PJM's capacity auction for the 2026 to 2027 delivery year, scheduled for July. The previous auction, for 2025 to 2026, cleared at sharply higher prices than the one before it, and the outcome has made capacity costs a political issue in several PJM states. Events like the June heat wave feed into how PJM assesses the value of capacity and the reliability contribution of different resources.
Demand response performance during the heat wave is relevant here. If large loads, including data centers, can reduce consumption during peaks, they can lower the capacity the system needs. PJM has said it needs all available generation of all types and will rely more on load flexibility tools in extreme conditions.
Why the evening hours mattered
The timing of the peaks, between 5 p.m. and 6 p.m., and the price spike at 7 p.m. on June 23 reflect a pattern that grid operators see increasingly in summer. Solar output, which helps during the middle of the day, falls in the late afternoon and evening while temperatures and air conditioning load stay high. Solar supplied 6% of PJM's generation at the June 23 peak, according to EIA, well below its midday contribution. The hours after the solar ramp-down are when the system leans most on gas peakers, oil units and demand response, and when prices are most likely to spike.
Battery storage can shift midday solar into those hours, and PJM's queue includes a large volume of storage projects. For now, battery capacity in PJM remains small relative to systems such as California and Texas, where batteries have become a major source of evening supply during heat waves.
What to watch
The rest of the summer will test whether PJM's resources hold up through longer or hotter episodes. July and August typically bring the highest temperatures in the region. The capacity auction results, due in late July, will show how the market values resources under tightening conditions. For the longer term, PJM's interconnection queue reforms, the pace of new generation coming online and the speed of data center connections will determine how much margin the system has in future heat waves.