<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-05-15</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001507957</issuerCik>
        <issuerName>Ideal Power Inc.</issuerName>
        <issuerTradingSymbol>IPWR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001535264</rptOwnerCik>
            <rptOwnerName>AWM Investment Company, Inc.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>527 MADISON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2600</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-05-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>535000</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.75</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1681935</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Limited Partnerships</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">AWM Investment Company, Inc., a Delaware Corporation (AWM), is the investment adviser to Special Situations Fund III QP, L.P. (QP), Special Situations Cayman Fund, L.P. (CAYMAN), Special Situations Technology Fund, L.P. (Tech) &amp; Special Situations Technology Fund II, L.P. (T2) &amp; together with QP, CAYMAN, &amp; Tech, the Funds).  As the investment adviser to the Funds, AWM holds sole voting &amp; investment power over 845,924 shares of Common Stock of the Issuer (the Shares) held by QP, 245,901 Shares held by CAYMAN, 87,944 Shares held by Tech &amp; 502,166 Shares held by T2.</footnote>
        <footnote id="F2">Austin W. Marxe (Marxe), David M. Greenhouse (Greenhouse) &amp; Adam C. Stettner (Stettner) are the controlling principals of AWM. The reporting person disclaims beneficial ownership of the Shares, except to the extent of its pecuniary interest therein. This report shall not be deemed an admission that the reporting person is the beneficial owner of the securities reported herein for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose.</footnote>
        <footnote id="F3">This is a weighted average price.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Adam Stettner</signatureName>
        <signatureDate>2015-05-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
