Why surplus interconnection could be one of Wisconsin’s most practical clean energy tools
In Portage, Wisconsin, there is a set of wires that has been carrying electricity for decades.
They connect a coal-fired power plant called Columbia Energy Center to the regional grid. Those wires run to a substation, and that substation connects to transmission lines that carry power to homes and businesses across the state.
Columbia is scheduled to retire. But those wires are not going anywhere.
That is the idea behind surplus interconnection, and it is one of the more practical and underappreciated tools in the clean energy transition.
How It Works
When a power plant connects to the electric grid, utility engineers size everything for that plant’s full output. The wires, the transformers, the substation, all of it. That infrastructure does not disappear when a plant retires, isn’t running, or scales back. The available capacity is still there, even if nothing is using it.
Surplus interconnection lets a new clean energy project plug into that existing connection instead of building a new one from scratch.
Think of it like a parking lot at a stadium that only fills up on game days. Most of the time, those spaces sit empty. Surplus interconnection lets new projects use that empty space, instead of paving a brand new lot somewhere else.
There is one important rule. You cannot park more cars in the lot than it was built to hold. The total electricity moving across the connection point cannot exceed what the original plant was approved to put on the system.
This is technology-neutral. Solar, wind, storage, existing fossil fuel plants, or any combination of them can share an existing interconnection, as long as the total output stays within the approved limit. It is about reusing the connection, not about choosing one technology over another.
Where the Opportunities Are
Retiring coal plants are a natural place to start. Columbia is a clear example of this principle. Alliant Energy is building a long-duration battery right next to the existing coal facility, on the same site, using the same grid connection. The Public Service Commission approved the project in 2025, construction begins this year, and once it is online, it will be able to discharge clean power for hours at a stretch.
Peaker plants tell a similar story. These are gas-fired facilities that utilities run only during periods of peak demand, often hot summer afternoons when air conditioners are running across the state. A peaker might only operate a few hundred hours a year, which means its grid connection sits idle most of the time. Co-locating solar, wind, or storage at a peaker means putting that connection to work the rest of the year. The peaker stays in place as backup for the rare hours when it is genuinely needed.
The same logic applies to existing renewables. A solar plant’s grid connection is sized for its peak output, which only happens for a few hours of sunny midday. A wind project often generates most at night, when demand is lower. Either way, the connection has room to spare much of the time. Adding storage, or pairing solar with wind to fill complementary hours, lets the project use that headroom and deliver clean power through the same connection when it is needed most.
This is already happening in Wisconsin. Solar developers across the state are pursuing battery additions at existing utility-scale solar projects.
Why It Matters Now
Wisconsin is about to see a major surge in electricity demand, driven in large part by new data center development. Meeting that demand by building new transmission lines and power plants, both of which take 5 or more years, could take longer than we would like.
Surplus interconnection provides an additional tool to help us meet the timeline required for projected load growth. Projects that reuse an existing connection point can often be built in two to three years. Though not a silver bullet for meeting energy demand in Wisconsin, it gives us an opportunity to meet this increase in demand without setting us back years on our clean energy goals.
MISO Has Already Given the Green Light
Wisconsin sits inside MISO, the Midcontinent Independent System Operator. MISO is the regional grid operator that manages the bulk power system across fifteen states, and it has explicitly built surplus interconnection into its rulebook. MISO’s own guidance steers developers who want to add batteries at existing plants toward surplus interconnection as the appropriate route. In short, the regional path is already open. What MISO does not do is require any utility to go looking for these opportunities. That is left to the states.
Other states are stepping into this space. Virginia recently passed the first-in-the-nation Facilitating Access to Surplus Transmission Act (FAST Act). Virginia sits in a different Regional Transmission Organization, but the underlying mechanism is the same. The law requires regulated utilities to inventory sites where they have unused interconnection capacity and run competitive solicitations to fill that capacity with new solar and storage. The stated goal is to cut interconnection timelines from years to months and avoid major new transmission build-outs that would otherwise land on customer bills
The tools exist. What is missing here is a consistent practice of looking for these opportunities.
Looking Ahead
Columbia is not the only Wisconsin plant on the way out. Oak Creek, Edgewater, and units at Weston are all scheduled to retire or convert in the next several years. Each one is a high-capacity connection point that could host new solar, wind, storage, or a mix of them. Wisconsin’s growing fleet of utility-scale solar and wind projects offers another set of opportunities for adding complementary generation and storage without new grid infrastructure, and the peakers across the state are solid candidates as well.
Surplus interconnection deserves more deliberate attention. The grid we already have is more valuable than we sometimes give it credit for. Reusing it well is how we get clean energy online faster and protect ratepayers from paying twice.