New England Grid Maintains Reliability Amidst Decade-High June Heatwave Demand Peak
Key Insights
ISO New England successfully managed a capacity deficiency during a June 24 heatwave, maintaining grid reliability despite high electricity demand.
Electricity demand peaked at 26,024 MW, the highest since 2013, driven by extreme heat and humidity across the region.
System operators activated reserve resources and reduced exports, implementing a Power Caution and Energy Emergency Alert Level 1, without requiring public conservation.
Behind-the-meter photovoltaics played a crucial role, reducing the actual peak demand by over 2,000 MW and shifting its timing by several hours.
ISO New England system operators successfully navigated a significant capacity deficiency on Tuesday, June 24, 2025, maintaining grid reliability during a regional heatwave that pushed electricity demand to its highest point in over a decade. The event saw consumer demand unofficially peak at 26,024 megawatts (MW) between 6 and 7 p.m., driven by average temperatures of 95.6° Fahrenheit and high humidity across New England. This peak, while substantial, remained below the region’s all-time record of 28,130 MW set in August 2006, yet it was largely consistent with the ISO’s pre-season summer forecast.
Despite the robust demand, approximately 2,550 MW of generating capability was unavailable, leading to a shortfall in operating reserve requirements. In response, beginning at 5:22 p.m., ISO New England initiated well-established procedures. These measures included the activation of reserve resources and a reduction in electricity exports to neighboring regions. The ISO also implemented a Power Caution and declared an Energy Emergency Alert Level 1 (EEA 1), the lowest of three alert levels, signaling heightened awareness without requiring public conservation efforts. The short duration of the deficiency and the effective deployment of reserve assets negated the need for consumer appeals.
Conditions on the system improved later in the evening as demand subsided and some generators returned to service. By 9 p.m., ISO New England canceled both the Power Caution and EEA 1 declarations. Prior to the event, at 9 p.m. on Monday, June 23, system operators had issued a precautionary alert, advising market participants to suspend any testing or maintenance that could impact grid reliability, a measure that was rescinded 48 hours later.
During the peak demand hour, carbon-emitting resources supplied approximately 74% of the total energy consumed. Natural gas, a foundational fuel for the region, consistently produced nearly 12,000 MW throughout the day. Nuclear resources contributed an average of 3,350 MW, while oil and hydro resources significantly ramped up their output to meet the evening peak. Critically, ISO estimates indicate that without the substantial contribution from behind-the-meter photovoltaics (BTM PV), the demand peak would have occurred three hours earlier, around 3:40 p.m., and reached 28,461 MW—more than 2,000 MW higher than the actual recorded peak. At that earlier time, non-carbon-emitting resources, encompassing BTM PV, nuclear, hydro, wind, telemetered solar, battery storage, and demand response, collectively provided a maximum of about 40% of the region’s energy.