ATC owns and operates electric transmission facilities primarily in Wisconsin. MGE received an interest in ATC when it, like other Wisconsin electric utilities, contributed its electric transmission facilities to ATC, as required by Wisconsin law. That interest is presently held by MGE Transco, a subsidiary of MGE Energy. ATC Holdco was formed by several members of ATC, including MGE Energy, to pursue electric transmission development and investments outside of Wisconsin. The ownership interest in ATC Holdco is held by MGEE Transco, a subsidiary of MGE Energy.

MGE Transco and MGEE Transco have accounted for their investments in ATC and ATC Holdco, respectively, under the equity method of accounting. Equity earnings from investments are recorded as "Other income" on the consolidated statements of income of MGE Energy. MGE Transco recorded the following amounts related to its investment in ATC:

MGE receives transmission and other related services from ATC. During the three months ended March 31, 2026, and 2025, MGE recorded $11.2 million and $10.2 million, respectively, for transmission service. MGE also provides a variety of

operational, maintenance, and project management work for ATC, which is reimbursed by ATC. As of March 31, 2026, and December 31, 2025, MGE had a receivable due from ATC of $3.0 million and $2.5 million, respectively. The receivable is primarily related to transmission interconnection activities at the renewable generation sites. MGE will be reimbursed for these costs after the new generation assets are placed into service.