INTERSTATE POWER AND LIGHT COMPANY (an Iowa corporation) 3.65% SENIOR DEBENTURES DUE 2020 PURCHASE AGREEMENT Dated: August 23, 2010Purchase Agreement • August 24th, 2010 • Alliant Energy Corp • Electric & other services combined • New York
Interstate Power and Light Company, an Iowa corporation (the “Company”), confirms its agreement with Banc of America Securities LLC (“BOA”) and Wells Fargo Securities, LLC (“Wells Fargo”), and each of the other underwriters named in Schedule A hereto (collectively, the “Underwriters”, which term shall also include any underwriter substituted as hereinafter provided in Section 10 hereof), for whom BOA and Wells Fargo are acting as representatives (in such capacity, the “Representatives”), with respect to the issue and sale by the Company and the purchase by the Underwriters, acting severally and not jointly, of the respective principal amounts set forth on Schedule A of $200,000,000 aggregate principal amount of the Company’s 3.65% Senior Debentures due 2020 (the “Securities”). The Securities will be issued pursuant to an indenture dated as of August 20, 2003 (the “Base Indenture”) between the Company and The Bank of New York Mellon Trust Company, N.A. (f/k/a J.P. Morgan Trust Company,
