Opinion Letter on S-3 Registered Securities – Alliant Energy Corp.

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CRAVATH, SWAINE & MOORE LLP

[New York Office]

December 16, 2011

Alliant Energy Corporation

Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as counsel for Alliant Energy Corporation, a Wisconsin
corporation (the “Company”), in connection with the filing of the Registration
Statement on Form S-3 (the “Registration Statement”) with the Securities and
Exchange Commission (the “Commission”) under the Securities Act of 1933, as
amended (the “Securities Act”), relating to the registration under the
Securities Act and the proposed issuance and sale from time to time pursuant to
Rule 415 under the Securities Act of: (i) shares of common stock of the Company,
$.01 par value per share (the “Common Stock”), and related Common Share Purchase
Rights (the “Rights”); (ii) debt securities of the Company, which may be
convertible into Common Stock (the “Debt Securities”); (iii) warrants for the
purchase of Debt Securities, Common Stock or other securities (the “Warrants”);
(iv) stock purchase contracts as described in the Registration Statement (the
“Stock Purchase Contracts”); and (iv) stock purchase units as described in the
Registration Statement (the “Stock Purchase Units”). The Common Stock and the
attached Rights, the Debt Securities, the Warrants, the Stock Purchase Contracts
and the Stock Purchase Units are referred to herein collectively as the “Offered
Securities”.

Unless otherwise provided in any prospectus supplement forming a part of the
Registration Statement relating to a particular series of the Debt Securities,
the Debt Securities will be issued under an Indenture (the “Indenture”) dated
September 30, 2009, between the Company and Wells Fargo Bank, National
Association, as trustee (the “Trustee”), filed as an exhibit to the Registration
Statement.

In connection with this opinion, we have examined originals, or copies
certified or otherwise identified to our satisfaction, of such corporate
records, certificates of corporate officers and government officials and such
other documents as we have deemed necessary or appropriate for the purposes of
this opinion. As to various questions of fact material to this opinion, we have
relied upon representations of officers or directors of the Company and
documents furnished to us by the Company without independent verification of
their accuracy. We have also assumed the genuineness of all signatures, the
authenticity of all documents submitted to us as originals and the conformity to
authentic original documents of all documents submitted to us as copies.

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Based upon and subject to the foregoing, and assuming that (i) the
Registration Statement and any supplements and amendments thereto (including
post-effective amendments) will have become effective and will comply with all
applicable laws; (ii) the Registration Statement will be effective and will
comply with all applicable laws at the time the Offered Securities are offered
or issued as contemplated by the Registration Statement; (iii) a prospectus
supplement will have been prepared and filed with the Commission describing the
Offered Securities offered thereby and will comply with all applicable laws;
(iv) all Offered Securities will be issued and sold in compliance with all
applicable Federal and state securities laws and in the manner stated in the
Registration Statement and the appropriate prospectus supplement; (v) none of
the terms of any Offered Security to be established subsequent to the date
hereof, nor the issuance and delivery of such Offered Security, nor the
compliance by the Company with the terms of such Offered Security will violate
any applicable law or will result in a violation of any provision of any
instrument or agreement then binding upon the Company or any restriction imposed
by any court or governmental body having jurisdiction over the Company; (vi) a
definitive purchase, underwriting or similar agreement and any other necessary
agreement with respect to any Offered Securities offered or issued will have
been duly authorized and validly executed and delivered by the Company and the
other parties thereto; and (vii) any Offered Securities issuable upon
conversion, exchange or exercise of any Offered Security being offered or issued
will be duly authorized, created and, if appropriate, reserved for issuance upon
such conversion, exchange or exercise, we are of opinion as follows:

1. With respect to the Debt Securities to be issued under the Indenture,
assuming that the Company and the Trustee have duly authorized, executed and
delivered the Indenture, when (A) the Trustee has been qualified to act as
trustee under the Indenture, (B) the Indenture has been qualified under the
Trust Indenture Act of 1939, as amended, (C) the Board of Directors of the
Company or a duly constituted and acting committee thereof (such Board of
Directors or committee thereof being hereinafter referred to as the “Board”) has
taken all necessary corporate action to approve the issuance and terms of a
particular series of such Debt Securities, the terms of the offering thereof and
related matters, and (D) such Debt Securities have been duly executed,
authenticated, issued and delivered in accordance with the provisions of the
Indenture and the applicable definitive purchase, underwriting or similar
agreement approved by the Board upon payment of the consideration therefor
provided for therein, such Debt Securities will constitute valid and binding
obligations of the Company, enforceable against the Company in accordance with
their terms (subject to applicable bankruptcy, insolvency, reorganization,
fraudulent conveyance, moratorium or other similar laws affecting creditors’
rights generally from time to time in effect and subject to general principles
of equity, including concepts of materiality, reasonableness, good faith and
fair dealing, regardless of whether such enforceability is considered in a
proceeding in equity or at law).

2. With respect to the Warrants, when (A) the Board has taken all necessary
corporate action to approve the creation of and the issuance and terms of the
Warrants, the terms of the offering thereof and related matters, (B) a warrant
agreement or agreements relating to the Warrants have been duly authorized and
validly executed and delivered by the Company, the warrant agent appointed by
the Company and each other

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party thereto and (C) the Warrants or certificates representing the Warrants
have been duly executed, countersigned, registered and delivered in accordance
with the appropriate warrant agreement or agreements and the applicable
definitive purchase, underwriting or similar agreement approved by the Board
upon payment of the consideration therefor provided for therein, the Warrants
will constitute valid and binding obligations of the Company, enforceable
against the Company in accordance with their terms (subject to applicable
bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or
other similar laws affecting creditors’ rights generally from time to time in
effect and subject to general principles of equity, including concepts of
materiality, reasonableness, good faith and fair dealing, regardless of whether
such enforceability is considered in a proceeding in equity or at law).

3. With respect to the Stock Purchase Contracts, when (A) the Board has taken
all necessary corporate action to approve and establish the terms of the Stock
Purchase Contracts and to authorize and approve the issuance thereof, the terms
of the offering and related matters, (B) a purchase agreement or agreements
relating to the Stock Purchase Contracts have been duly authorized and validly
executed and delivered by the Company and each other party thereto and (C) the
Stock Purchase Contracts have been duly executed and delivered in accordance
with the purchase agreement and the applicable definitive purchase, underwriting
or similar agreement approved by the Board upon payment of the consideration
therefor provided for therein, the Stock Purchase Contracts will constitute
valid and binding obligations of the Company, enforceable against the Company in
accordance with their terms (subject to applicable bankruptcy, insolvency,
reorganization, fraudulent conveyance, moratorium or other similar laws
affecting creditors’ rights generally from time to time in effect and subject to
general principles of equity, including concepts of materiality, reasonableness,
good faith and fair dealing, regardless of whether such enforceability is
considered in a proceeding in equity or at law).

4. With respect to the Stock Purchase Units, when (A) the Board has taken all
necessary corporate action to approve and establish the terms of the Stock
Purchase Units and to authorize and approve the issuance thereof, the terms of
the offering and related matters, (B) a purchase agreement or agreements
relating to the Stock Purchase Units have been duly authorized and validly
executed and delivered by the Company and each other party thereto and (C) the
Stock Purchase Units have been duly executed and delivered in accordance with
the purchase agreement and the applicable definitive purchase, underwriting or
similar agreement approved by the Board upon payment of the consideration
therefor provided for therein, the Stock Purchase Units will constitute valid
and binding obligations of the Company, enforceable against the Company in
accordance with their terms (subject to applicable bankruptcy, insolvency,
reorganization, fraudulent conveyance, moratorium or other similar laws
affecting creditors’ rights generally from time to time in effect and subject to
general principles of equity, including concepts of materiality, reasonableness,
good faith and fair dealing, regardless of whether such enforceability is
considered in a proceeding in equity or at law).

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We express no opinion herein as to any provision of the Indenture or the Debt
Securities that (a) relates to the subject matter jurisdiction of any Federal
court of the United States of America, or any Federal appellate court, to
adjudicate any controversy related thereto, (b) contains a waiver of an
inconvenient forum, (c) relates to the waiver of rights to jury trial or (d)
provides for indemnification, contribution or limitations on liability. We also
express no opinion as to (i) the enforceability of the provisions of the
Indenture or the Debt Securities to the extent that such provisions constitute a
waiver of illegality as a defense to performance of contract obligations or any
other defense to performance which cannot, as a matter of law, be effectively
waived or (ii) whether a state court outside the State of New York or a Federal
court of the United States would give effect to the choice of New York law
provided for therein.

Courts in the United States have not customarily rendered judgments for money
damages denominated in any currency other than United States dollars. Section
27(b) of the Judiciary Law of the State of New York provides, however, that a
judgment or decree in an action based upon an obligation denominated in a
currency other than United States dollars shall be rendered in the foreign
currency of the underlying obligation and converted into United States dollars
at the rate of exchange prevailing on the date of the entry of the judgment or
decree. We express no opinion as to whether a Federal court would render a
judgment other than in United States dollars.

We are admitted to practice only in the State of New York and express no
opinion as to matters governed by any laws other than the laws of the State of
New York and the Federal laws of the United States of America. Our opinions set
forth above, insofar as they involve matters of the laws of the State of
Wisconsin, are qualified to the extent that we have relied upon the opinion
dated the date hereof of Jake C. Blavat, Senior Attorney of the Company (the
“Company Opinion”), and we have assumed, without independent investigation, the
correctness of, and take no responsibility for, the Company Opinion.

We are aware that we are referred to under the heading “Legal Matters” in the
prospectus forming a part of the Registration Statement and that we may be
referred to under a similar heading in a prospectus supplement filed after the
effective date of the Registration Statement. We hereby consent to such use of
our name therein and the filing of this opinion as Exhibit 5 to the Registration
Statement. In giving this consent, we do not hereby admit that we are within the
category of persons whose consent is required under Section 7 of the Securities
Act or the Rules and Regulations of the Commission promulgated thereunder.

Very truly yours, /s/ Cravath, Swaine & Moore LLP

Alliant Energy Corporation

4902 North Biltmore Lane

Madison, WI 53718

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