ISO New England (ISO-NE) operates approximately 9,000 miles of high-voltage transmission and roughly 30 GW of installed generation capacity across six states. The region is connected to neighboring systems through 14 tie lines, with about 7% of its energy supplied by imports in 2025. A new HVDC line to Québec was placed into service in late 2025.
Regional electrification of heating and transportation is expected to drive sustained increases in demand. At the same time, most of the New England states are pursuing reductions in electric-sector carbon emissions. The states’ collective goal is an 80% reduction below 1990 emissions levels by 2050, a target ISO-NE models in its forward-looking economic studies.
With roughly $13 billion invested in transmission since 2002, and hundreds of millions of additional reliability investments planned in the near term, ISO-NE must also plan a transmission system that allows for electrification and deep decarbonization while maintaining reliability. These complexities require a modeling framework capable of co-optimizing generation expansion, transmission constraints, policy targets, and reliability metrics within a single integrated analytical environment. To help support this challenge, ISO-NE implemented Energy Exemplar’s PLEXOS® as its core long-term economic planning platform.
The New England grid’s transition presents three interrelated challenges:
Weather Variability and Seasonal Shift
As wind and solar generation grow, the power system becomes increasingly sensitive to weather. Modeling indicates that as electrified heating increases, New England is likely to transition from a summer-peaking system to a winter-peaking one.
Capacity expansion results are highly dependent on weather assumptions, and modeling different historical weather years produces variable future system needs. Long-term build forecasts are highly sensitive to winter peak demand assumptions and renewable output variability, which materially affects both infrastructure investment and total system cost.
Reliability in a Decarbonized System
As emissions decline, seasonal operating patterns change. By the modeled year 2040, when the region approaches 80–85% decarbonization, spring and fall periods are nearly carbon-free due to strong renewable output and moderate demand.
By 2050, fossil resources operate on relatively few days, but when they are needed, they run near full capacity. Dispatchable resources remain essential during extreme conditions, even if they sit idle for most of the year.
This creates a structural reliability challenge: maintaining sufficient firm capacity for rare but critical events in a grid dominated by variable generation.
Increasing Marginal Costs of Decarbonization
Between 2035 and 2045, a large portion of the states’ emissions goals can be achieved cost-effectively through land-based wind and solar. Over time, however, the cost to eliminate each additional ton of carbon rises, suggesting practical limits to how much additional variable renewable capacity can be added economically.
Beyond 2045, deeper carbon reductions require greater reliance on storage and advanced dispatchable technologies, which increase system costs.
ISO-NE began using PLEXOS® as part of the 2022–2024 Economic Planning for the Clean Energy Transition (EPCET) study, introducing capacity expansion modeling into its long-term planning process. The platform now serves as a co-optimized analytical backbone, enabling ISO-NE to evaluate generation, transmission, and reliability decisions within a single modeling environment.
Structured Scenario Analysis
Each scenario serves a distinct analytical purpose, enabling ISO-NE to test system performance across time horizons and policy assumptions. Long-term studies are conducted under four main scenarios:
The policy scenario assesses multiple pathways to state emissions goals.
The Importance of Dispatchable Zero-Carbon Resources
ISO-NE’s long-term modeling indicates that dispatchable zero-carbon resources materially reduce total system build requirements. When advanced dispatchable technologies such as small modular nuclear reactors (SMRs) and long-duration storage are included as candidate resources, overall installed capacity requirements in 2050 decline significantly, compared to scenarios relying primarily on renewables.
In ISO-NE’s most recent economic study, the 2024 Economic Study, the inclusion of dispatchable zero-carbon resources reduced total installed capacity needs by 2050 from approximately 96 GW to roughly 80 GW, driven primarily by SMR additions. These results demonstrate that firm, dispatchable capacity can reduce the renewable generation and storage required to meet reliability and decarbonization goals.
The findings highlight the role of dispatchability in moderating system buildout, particularly in the later stages of decarbonization when maintaining reliability during extreme conditions becomes increasingly challenging.
Offshore Wind and Resource Mix Diversity
Offshore wind is central to New England’s future resource mix. The 2024 Economic Study evaluated a scenario without any new offshore wind, and one that increased its assumed capital cost by 20% (fixed-bottom) and 50% (floating).
Results showed that:
ISO-NE’s first long-term transmission study, the 2050 Transmission Study, informed a competitive transmission request-for-proposals (RFP) process initiated by the New England States Committee on Electricity (NESCOE).
The RFP sought to:
ISO-NE is using PLEXOS® to evaluate system performance for several of the benefits of each proposed project. The five quantified benefits of avoided generation and transmission investment, production cost savings and congestion reductions, and reduced expected unserved energy are combined into a benefit-cost ratio (BCR). A BCR of >1.0 serves as the economic threshold to move forward with a proposal unless no project delivers a BCR over 1.0.
ISO-NE has also adapted PLEXOS® to perform probabilistic reliability screening for long-term transmission planning.
The model runs 20 historic weather years and 100 forced outage draws, producing 2,000 system realizations. Historical outage data and mesoscale weather backcasting are incorporated directly, and unserved energy is assigned a high penalty cost to prioritize reliability outcomes.
This allows for the assessment of expansion, transmission, and reliability within a consistent modeling environment.
Maintaining reliability through New England’s clean energy transition requires balancing electrification-driven load growth, weather-dependent generation, and rising marginal decarbonization costs.
By adopting PLEXOS® as an integrated platform, ISO New England has strengthened its ability to evaluate long-term infrastructure decisions through increased uncertainty.