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ISO New England continues to expand its ability to forecast the growth and impact of “behind-the-meter” resources, including solar panels and batteries.
These kinds of resources are not visible to the ISO-NE control room and do not participate in the region’s wholesale electricity markets. It’s important to keep track of them because they affect both how and when consumers use electricity from the grid.
One way the ISO keeps track is with its annual Forecast Report of Capacity, Energy, Loads, and Transmission (CELT Report), a foundational resource for system planning and reliability studies. The 2026 edition was published May 1. Additional detail on BTM and other kinds of distributed energy resources is presented in the Final 2026 DER Forecast.
This is the first time the CELT Report has included forecast data for behind-the-meter battery energy storage systems (BTM BESS). Specifically, these are systems with nameplate capacity of less than 1 megawatt (MW) that are co-located with rooftop solar panels. The ISO’s innovative load forecasters have been developing methods to account for BTM BESS over the last few years.
BTM BESS is an emerging technology, and the ISO’s forecast indicates it’s likely to remain a small portion of the region’s electricity landscape over the next decade. Factors that could lead to stronger growth in this area include declining technology costs and new incentive programs.
Though the data are incomplete, surveys of retail electricity providers indicate there is about 111 MW worth of BTM BESS that is less than 1 MW in size and co-located with PV across the region today. The ISO expects about 173 MW of BTM BESS to be added over the next 10 years, most of it in Massachusetts.
Meanwhile, the ISO expects continued growth in behind-the-meter photovoltaics (BTM PV).
The BTM PV forecast is somewhat more conservative than in previous years, reflecting the expiration of the federal Investment Tax Credit, which incentivized residential and commercial solar installations. State-level incentives, however, will continue to drive growth of behind-the-meter and other solar resources.
New England’s BTM PV capacity is expected to reach about 5,500 MW by the end of 2026 and 7,950 MW in 2035.
BTM PV is projected to reduce consumption of grid electricity by 7,056 gigawatt-hours (GWh) this year, rising to 10,197 GWh in 10 years. In other words, without BTM PV, annual energy use would be about 6% higher in 2026 and 8% higher in 2035.
The impact of BTM PV on peak demand — the highest amount of electricity used in a single hour — varies by season.
Under average conditions, BTM PV will reduce the peak by an average of 1,936 MW each summer through 2035, the forecast indicates. Although in past years the summer peak typically occurred in the afternoon, widespread adoption of BTM PV has pushed the peak toward sunset. This means that even as more BTM PV comes on line, its impact on summer peaks will not significantly increase.
In winter, demand tends to be highest after sunset, and so BTM PV traditionally has not reduced winter peaks. But after 2030, winter peaks are increasingly likely to happen in the morning when the sun is rising. That’s because wider adoption of heat pumps in homes and businesses is expected to create greater electricity use for heating near the start of the business day. The forecast indicates BTM PV will reduce peak demand by 316 MW in winter 2035/2036.
Winter days in the mid-2030s may see a mix of morning and evening peaks. This could present a challenge for BTM BESS. Retail “peak shaving” programs instruct batteries to discharge around the times when demand for grid electricity is expected to be highest. Uncertainty arises when demand reaches similar levels in both morning and evening. For example, batteries activated in anticipation of a morning peak may not have any energy left to discharge if the evening peak turns out to be higher.
The ISO’s BTM BESS forecast accounts for this uncertainty. It indicates a 2035/2026 winter peak reduction from BTM BESS of 0 MW, reflecting a 50% chance that BTM BESS dispatch misses the peak load during winter peak conditions. For the summer of 2035, the peak reduction value is 124 MW.
BTM BESS is expected to increase annual energy use by 5 GWh in 2035. In contrast to behind-the-meter solar, BTM BESS increases overall demand for grid electricity. That’s because a portion of the energy used to charge a battery is always lost. Essentially, batteries always put out less energy than they take in. Batteries play an important role, however, in shifting demand away from peak hours by storing energy produced at times of low demand.
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