Three Mile Island, Susquehanna and 26,500 MW of Texas Requests: Data Centers Are Shopping for Firm Power Before the Grid Can Plan for Them

Two contracts signed in 2024 changed how the US power sector talks about data centers. In March, Amazon Web Services agreed to buy up to 960 megawatts of capacity from Talen Energy's Susquehanna nuclear plant in Pennsylvania. In September, Constellation Energy announced a 20-year power purchase agreement to supply Microsoft data centers in the mid-Atlantic from Three Mile Island Unit 1, a reactor that returned to service in 1985 after the 1979 accident at the adjacent Unit 2 and was retired in 2019 because its then owner said it was no longer profitable.

Nuclear plants are costly to build but cheap to run, a single reactor typically has 800 MW or more of capacity, and they produce electricity without directly emitting carbon dioxide. The Susquehanna deal is worth reading closely, because its structure reveals how unsure even the buyers are about their own demand. Rather than taking the full 960 MW at once, AWS will ramp up its share in 120 MW increments over several years, and it has a one-time option to cap its commitment at 480 MW.

For grid planners, it is a signal that headline numbers for data center demand should be treated as ranges, not forecasts. The Electric Reliability Council of Texas, which manages 90% of the state's load, set up a program in mid-2022 to approve large flexible load customers, defined as those with expected peak demand of 75 MW or more. For its forecasts, the EIA assumes that ERCOT will have approved 9,500 MW by the end of 2025, 73% more than currently.

Virginia leads, driven by Dominion Energy's territory, where 94 new data centers have connected since 2019, drawn by a dense fiber backbone and four subsea cables. At the same time, the data center may continue to rely on the grid for backup, for voltage support and for transmission services, even if it draws power directly from the plant. The restart at Three Mile Island is a different case, because it adds capacity that was not on the system.