SECURITIES AND EXCHANGE COMMISSION FORM …

1 1 United States SECURITIES AND EXCHANGE COMMISSION Washington, 20549 FORM 10-K [X] Annual Report Pursuant to Section 13 or 15(d) of the SECURITIES EXCHANGE Act of 1934 For the fiscal year ended: December 31, 2017 [ ] Transition report pursuant to Section 13 or 15(d) of the SECURITIES EXCHANGE Act of 1934 For the transition period from _____ to _____ COMMISSION File No. Name of Registrant, State of Incorporation, Address of Principal Executive Offices, and Telephone No. IRS Employer Identification No. 000-49965 MGE Energy, Inc. (a Wisconsin Corporation) 133 South Blair Street Madison, Wisconsin 53788 (608) 252-7000 39-2040501 000-1125 Madison Gas and Electric Company (a Wisconsin Corporation) 133 South Blair Street Madison, Wisconsin 53788 (608) 252-7000 39-0444025 SECURITIES REGISTERED PURSUANT TO SECTION 12(b) OF THE ACT: Title of Class Name of Each EXCHANGE on which Registered MGE Energy, Inc.

2 Common Stock, $1 Par Value Per Share The Nasdaq Stock Market SECURITIES REGISTERED PURSUANT TO SECTION 12(g) OF THE ACT: Title of Class Madison Gas and Electric Company .. Common Stock, $1 Par Value Per Share Indicate by checkmark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the SECURITIES Act. MGE Energy, Inc.. Yes [X] No [ ] Madison Gas and Electric Company .. Yes [X] No [ ] Indicate by checkmark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act. MGE Energy, Inc.. Yes [ ] No [X] Madison Gas and Electric Company .. Yes [ ] No [X] 2 Indicate by check mark whether the registrants (1) have filed all reports required to be filed by Section 13 or 15(d) of the SECURITIES EXCHANGE Act of 1934 during the preceding 12 months (or for such shorter period that the registrants were required to file such reports) and (2) have been subject to such filing requirements for the past 90 days.

3 Yes [X] No [ ] Indicate by check mark whether the registrants have submitted electronically and posted on their corporate Web sites, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T ( of this chapter) during the preceding 12 months (or for such shorter period that the registrants were required to submit and post such files): Yes [X] No [ ] Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrants' knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.

4 [ ] Indicate by check mark if the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See definition of "large accelerated filer," "accelerated filer," "smaller reporting company," and "emerging growth company" in Rule 12b-2 of the EXCHANGE Act: Large Accelerated Filer Accelerated Filer Non-accelerated Filer Smaller Reporting Company Emerging Growth Company MGE Energy, Inc..X Madison Gas and Electric Company .. X Indicate by checkmark whether the registrant is a shell company (as defined in Rule 12b-2 of the EXCHANGE Act).

5 MGE Energy, Inc.. Yes [ ] No [X] Madison Gas and Electric Company .. Yes [ ] No [X] The aggregate market value of the voting and nonvoting common equity held by nonaffiliates of each registrant as of June 30, 2017, was as follows: MGE Energy, Inc.. $2,225,394,029 Madison Gas and Electric Company .. $0 The number of shares outstanding of each registrant's common stock as of February 1, 2018, were as follows: MGE Energy, Inc.. 34,668,370 Madison Gas and Electric Company .. 17,347,894 DOCUMENTS INCORPORATED BY REFERENCE Portions of MGE Energy, Inc.'s definitive proxy statement to be filed on or before March 26, 2018, relating to its annual meeting of shareholders, are incorporated by reference into Part III of this annual report on Form 10-K.

6 Madison Gas and Electric Company meets the conditions set forth in General Instruction (I)(1)(a) and (b) of Form 10-K and is therefore omitting (i.) the information otherwise required by Item 601 of Regulation S-K relating to a list of subsidiaries of the registrant as permitted by General Instruction (I)(2)(b), (ii.) the information otherwise required by Item 6 relating to Selected Financial Data as permitted by General Instruction (I)(2)(a), (iii.) the information otherwise required by Item 10 relating to Directors and Executive Officers as permitted by General Instruction (I)(2)(c), (iv.) the information otherwise required by Item 11 relating to executive compensation as permitted by General Instruction (I)(2)(c), (v.)

7 The information otherwise required by Item 12 relating to Security Ownership of Certain Beneficial Owners and Management as permitted by General Instruction (I)(2)(c), and (vi.) the information otherwise required by Item 13 relating to Certain Relationships and Related Transactions as permitted by General Instruction (I)(2)(c). 3 Table of Contents Filing Format .. 4 Forward-Looking Statements .. 4 Where to Find More Information .. 4 Definitions, Abbreviations, and Acronyms Used in the Text and Notes of this Report .. 5 PART I.. 7 Item 1. Business.. 7 Item 1A. Risk Factors.. 13 Item 1B. Unresolved Staff Comments.. 19 Item 2. Properties.. 20 Item 3.

8 Legal Proceedings.. 21 Item 4. Mine Safety Disclosures.. 21 PART II.. 22 Item 5. Market for Registrants' Common Equity, Related Stockholder Matters, and Issuer Purchases of Equity SECURITIES .. 22 Item 6. Selected Financial Data.. 25 Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations.. 26 Item 7A. Quantitative and Qualitative Disclosures About Market Risk.. 48 Item 8. Financial Statements and Supplementary Data.. 50 Item 9. Changes in and Disagreements with Accountants on Accounting and Financial Disclosure.. 104 Item 9A. Controls and Procedures.. 104 Item 9B. Other Information.. 104 PART III.. 105 Item 10. Directors, Executive Officers, and Corporate Governance.

9 105 Item 11. Executive Compensation.. 105 Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters.. 105 Item 13. Certain Relationships and Related Transactions, and Director Independence.. 105 Item 14. Principal Accounting Fees and Services.. 106 PART IV.. 107 Item 15. Exhibits and Financial Statement Schedules.. 107 Item 16. Form 10-K Summary.. 109 Signatures - MGE Energy, Inc.. 114 Signatures - Madison Gas and Electric Company .. 115 4 Filing Format This combined Form 10-K is being filed separately by MGE Energy, Inc. (MGE Energy) and Madison Gas and Electric Company (MGE). MGE is a wholly owned subsidiary of MGE Energy and represents a majority of its assets, liabilities, revenues, expenses, and operations.

10 Thus, all information contained in this report relates to, and is filed by, MGE Energy. Information that is specifically identified in this report as relating solely to MGE Energy, such as its financial statements and information relating to its nonregulated business, does not relate to, and is not filed by, MGE. MGE makes no representation as to that information. The terms "we" and "our," as used in this report, refer to MGE Energy and its consolidated subsidiaries, unless otherwise indicated. Forward-Looking Statements This report, and other documents filed by MGE Energy and MGE with the SECURITIES and EXCHANGE COMMISSION (SEC) from time to time, contain forward-looking statements that reflect management's current assumptions and estimates regarding future performance and economic conditions especially as they relate to economic conditions, future load growth, revenues, expenses, capital expenditures, financial resources, regulatory matters, and the scope and expense associated with future environmental regulation.