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Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ( Mark One ) Annual Report pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the fiscal year ended December 31 , 2020 or Transition Report pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION ( Exact name of registrant as specified in its charter ) Minnesota ( State or other jurisdiction of incorporation or organization ) 215 South Cascade Street , Box 496 , Fergus Falls , Minnesota ( Address of principal executive offices ) Registrant's telephone number , including area code : 866-410-8780 Securities registered pursuant to Section 12 ( b ) of the Act : Title of each class Trading Symbol ( s ) OTTR 27-0383995 ( I.R.S. Employer Identification No. ) 56538-0496 ( Zip Code ) Common Shares , par value $ 5.00 per share Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes Indicate by check mark whether the registrant ( 1 ) has 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 registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes No No No ✔ Accelerated Filer Smaller Reporting Company Name of each exchange on which registered The Nasdaq Stock Market LLC Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes ✔ No Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer or a smaller reporting company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . ( Check one ) : Large Accelerated Filer Non - Accelerated Filer Emerging Growth Company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . ✔ Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No The aggregate market value of common stock held by non - affiliates , computed by reference to the last sales price on June 30 , 2020 was $ 1,546,518,975 . Indicate the number of shares outstanding of each of the registrant's classes of common stock , as of the latest practicable date : 41,510,455 Common Shares ( $ 5 par value ) as of February 16 , 2021 . DOCUMENTS INCORPORATED BY REFERENCE The Registrant's definitive Proxy Statement for its 2021 Annual Meeting of Shareholders is incorporated by reference into Part III of this Form 10 - K .