How Eversource Used PLEXOS® to Put a Price Tag on Good Grid Planning

Eversource is New England's largest utility, serving more than 60% of the region's households and over 3.2 million electricity customers across Connecticut, Massachusetts, and New Hampshire. The company owns and operates roughly 4,500 miles of high-voltage transmission and 60,000 miles of distribution lines, all while working toward a corporate pledge to reach net zero emissions by 2050. A five-person team runs PLEXOS® for economic modeling and transmission strategy, and in 2025 that small team took on one of the most complex planning challenges New England has ever handed a transmission owner.

New England's Grid is Getting More Complicated, Not Less

New England's electricity market is restructured, so utilities like Eversource own transmission and distribution but not generation. That job belongs to independent generators, with ISO New England running the markets and planning for reliability. The region leans heavily on natural gas, nuclear, and imported hydropower. Its transmission network tops out at 345 kV - a modest ceiling given the terrain and space constraints across six states.

Electrification is reshaping demand too. As electric vehicles and electric heating systems spread, ISO New England expects the region's traditionally summer-peaking system to flip to a winter peak sometime after 2035, a shift that changes what "reliable" even means for a transmission planner. Add six states with different clean energy targets, most aiming for net zero by 2050, and Eversource is planning for a grid that has to satisfy engineering, economics, and public policy all at once.

A Solicitation with a New Set of Rules

In March 2025, the New England States Committee on Electricity (NESCOE) launched a public-policy-driven solicitation for new transmission in Maine, aimed at increasing transfer capability between northern and southern New England and making it easier for new renewable projects to interconnect.

The technical asks were substantial on their own:

• Boost the Surowiec South interface to 3,200 MW
• Raise the Maine-New Hampshire interface to 3,000 MW
• Accommodate 1,200 MW of new land-based wind

Proof of economic benefit, not just engineering soundness, made this solicitation different and considerably tougher as this requirement never applied to a New England transmission project before. Historically, these projects lived or died on reliability studies run through tools like PSS/E or GE MARS. This time, any solution that cleared the engineering bar still had to prove its value through a full economic evaluation, all within a six-month window that left little room to figure out a methodology as the project went along.

Three Models, Twenty Years of Answers

To meet that bar, Eversource built its economic case in PLEXOS® across three linked models spanning different time horizons. A long-term zonal capacity expansion model projected the future resource mix, including floating and fixed-bottom offshore wind, land-based wind, close to 45 GW of solar, and both 4-hour and 8-hour energy storage, letting the model decide where new capacity would get built. Those results fed into a 2035 short-term nodal production cost model to capture first-year market impacts, then into a 2054 short-term nodal model, run at hourly resolution across a full year, to estimate benefits over a 20-year horizon.

The metrics that mattered were

• Avoided capacity costs
• Reductions in production costs
• Reductions in unserved energy
• Reductions in line losses

This provided quantifiable apple-to-apples benefits that regulators and stakeholders could compare across competing proposals.

When Engineering Moves Twice a Week, the Model Has to Keep Up 

Running that kind of layered analysis is hard enough with a stable design. Eversource didn't have one. Engineering solutions changed roughly twice a week as the project team refined the transmission topology, and the economic model had to keep pace without waiting for a finished design, since the six-month clock made a "wait for final engineering, then evaluate" approach unworkable.

The team leaned on the PLEXOS® API to automate model updates rather than manually rebuilding new substations, interface limits, and line changes across separate models every time the engineering shifted. That alone saved enormous time across the roughly one hundred simulations the team ended up running.

Even automated, full production cost runs were too heavy to repeat for every minor tweak. Eversource built a multivariate regression model to estimate the economic impact of smaller interface changes, reserving full PLEXOS® runs for material design shifts. That approach kept leadership supplied with fast, credible numbers right up until the final week of the solicitation, when decisions about which projects to advance were still being made.

Multi-Value Planning Is the New Baseline

For a transmission and distribution utility built around engineering, running this level of economic analysis was new territory. Eversource's takeaway is that it shouldn't stay novel for long. Reliability, economics, and public policy all need to be modeled together and early, not bolted onto a project after the engineering is locked in. Skipping that step risks a costly misstep that every utility wants to avoid building or upgrading a line that ends up increasing curtailment or congestion instead of relieving it.

The work is also shaping how Eversource engages with state regulators across parts of New England, giving the company a clearer read on where renewable procurement is headed so it can plan interconnections that are reliable, efficient, and economical from the start. As long-term planning requirements gain traction across the industry, Eversource's PLEXOS®-driven approach may be a preview of how every transmission owner in the region will have to plan.