Amid the rapid growth of behind-the-meter (BTM) solar in New England, a capacity deficiency event demonstrated the significant benefits of solar resources, along with their limits in displacing fossil resources during peak load periods.

On June 24, extreme heat and humidity caused ISO-NE peak demand to surpass 26,000 MW at about 7 p.m., marking the region’s highest peak load since summer 2013. (See Extreme Heat Triggers Capacity Deficiency in New England.)

Without the contributions of BTM solar, ISO-NE estimates the peak would have reached over 28,400 MW at about 3:40 p.m. The 2,400-MW reduction in the region’s peak provided significant cost and reliability benefits to the grid. According to an analysis by the Acadia Center, “BTM solar avoided as much as roughly $19.4 million in costs on this single day by suppressing the overall price of wholesale electricity.”

In recent years, New England has seen annual additions of about 700 MW of BTM solar capacity, largely driven by state policy in Massachusetts and Connecticut. This has helped prevent load growth, pushed peaks later in the day and contributed to a growing duck curve and evening ramping requirements in the region. (See Growth of BTM Solar Drives Record-low Demand in ISO-NE.)

While BTM solar made a significant contribution to lowering the peak load, fossil resources continue to dominate the generation mix during the peak hour. ISO-NE estimates that carbon-emitting generation “provided about 74% of total energy consumed in the region during the peak” on June 24, including over 12,000 MW of natural gas generation, over 3,000 MW of oil generation and about 300 MW of coal generation.

The region’s reliance on fossil generation to keep the grid running was not without challenges — outages of large fossil units appear to have played a major role in triggering the deficiency event. Natural gas generation declined by about 1,000 MW immediately prior to ISO-NE’s declaration of a capacity deficiency, and the RTO estimates there were about 2,550 MW of generator outages and reductions at the time of the declaration.

The performance of BTM solar, coupled with fossil unit outages, has drawn attention from solar advocates. At a Massachusetts legislative hearing on June 25, several representatives of solar companies pointed to the benefits of solar during the deficiency when arguing against a proposal from Gov. Maura Healey (D) to reduce net metering compensation for new large solar facilities. (See Mass. Gov. Healey Introduces Energy Affordability Bill.)

“Solar … is the reason we didn’t have backouts and we didn’t have even higher prices and even higher emissions over the last few days,” said Jessica Robertson of New Leaf Energy.

However, incremental standalone solar capacity likely will have diminishing effects on peak loads in the coming years, as BTM solar has pushed peak periods into the evening, when solar production declines rapidly. ISO-NE’s 2025 Capacity, Energy, Loads and Transmission report estimates that increasing BTM solar will reduce the region’s gross summer peak by only an additional 144 MW by 2034.

In the wake of the capacity deficiency event, clean energy advocates made the case that increased energy storage capacity would have provided significant benefits during the peak.

“Had we had even more behind-the-meter solar paired with storage online, we could have potentially completely avoided that absurd price spike later in the evening,” said Kyle Murray of the Acadia Center at the June 25 hearing.

The Acadia Center wrote in its analysis of the event that there is “clear evidence that additional BTM battery energy storage would have been able to further reduce the overall cost to consumers by increasing flexibility and shifting the solar production later in the day, dampening the early evening peak prices.”

Consulting firm Power Advisory estimated that 1,000 MW of battery storage capacity could have reduced real-time LMPs by an average of over $100/MWh during the event, saving up to $17/kW. The firm also estimated that offshore wind would have reduced LMPs by $47/MWh, assuming a capacity factor of nearly 50% based on prevailing wind speeds.

While battery storage is in its infancy in the region, it is poised to grow quickly in the coming years, which would help to balance the production profile of storage. About 1,800 MW of energy storage cleared in ISO-NE’s capacity auction for the 2027/28 capacity commitment period, including 700 MW of new storage. Storage resources also account for 45% of the active projects in ISO-NE’s interconnection queue, totaling 18.4 GW in capacity.