<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>006200 - Disclosure - Acadia Unit 1 Transaction</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>1/1/2010 - 12/31/2010
</KeyName><CurrencySymbol /><contextRef><ContextID>c00052</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001089819</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2010-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2010-12-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Id="1" Label="12 Months Ended" /><Label Id="2" Label="Dec. 31, 2010" /></Labels></Column></Columns><Rows><Row><Id>2</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><Level>0</Level><ElementName>cnl_NotesToFinancialStatementsAbstract</ElementName><ElementPrefix>cnl</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole /><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>No definition available.</ElementDefenition><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Notes to the Financial Statements [Abstract]</Label></Row><Row><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_PropertyPlantAndEquipmentScheduleOfSignificantAcquisitionsAndDisposalsTextBlock</ElementName><ElementPrefix>us-gaap</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terselabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left" align="left"&gt;&lt;font style="DISPLAY: inline; FONT-WEIGHT: bold; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Cond, sans-serif"&gt;Note 19 &amp;#8212; Acadia Unit 1 Transaction &lt;/font&gt; &lt;div&gt;&lt;hr style="COLOR: black" align="left" width="100%" noshade="noshade" size="1" /&gt;&lt;/div&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;div style="DISPLAY: block; TEXT-INDENT: 0pt"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;On February 23, 2010, Cleco Power completed the acquisition of Acadia Unit 1 and half of Acadia Power Station&amp;#8217;s related common facilities.&amp;#160;&amp;#160;The significant terms of the transaction were:&lt;/font&gt;&lt;/div&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left" align="justify"&gt;&amp;#160;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;table cellspacing="0" cellpadding="0" width="100%"&gt;&lt;tr valign="top"&gt;&lt;td style="WIDTH: 27pt" align="right"&gt;&lt;div&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Wingdings"&gt;&amp;#167;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div style="MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;Cleco Power acquired Acadia Unit 1 and half of the common facilities for $304.0 million;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;table cellspacing="0" cellpadding="0" width="100%"&gt;&lt;tr valign="top"&gt;&lt;td style="WIDTH: 27pt" align="right"&gt;&lt;div&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Wingdings"&gt;&amp;#167;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div style="MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;Cleco Power recognized $78.4 million of deferred taxes on the transaction.&amp;#160;&amp;#160;For more information on the deferred taxes, see Note 9 &amp;#8212; &amp;#8220;Income Taxes;&amp;#8221;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;table cellspacing="0" cellpadding="0" width="100%"&gt;&lt;tr valign="top"&gt;&lt;td style="WIDTH: 27pt" align="right"&gt;&lt;div&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Wingdings"&gt;&amp;#167;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div style="MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;Acadia recognized a gain of $82.0 million;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;table cellspacing="0" cellpadding="0" width="100%"&gt;&lt;tr valign="top"&gt;&lt;td style="WIDTH: 27pt" align="right"&gt;&lt;div&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Wingdings"&gt;&amp;#167;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div style="MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;APH received $6.8 million from third parties in return for APH&amp;#8217;s indemnification against the third parties 50% share of Acadia&amp;#8217;s liabilities and other obligations related to the Cleco Power transaction; &lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;table cellspacing="0" cellpadding="0" width="100%"&gt;&lt;tr valign="top"&gt;&lt;td style="WIDTH: 27pt" align="right"&gt;&lt;div&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Wingdings"&gt;&amp;#167;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div style="MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;Cleco Power and the parties executed the definitive agreements in 2009, and received LPSC and FERC approvals for the transaction in January 2010 and February 2010, respectively; and&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="DISPLAY: block; MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt; TEXT-ALIGN: left"&gt;&lt;table cellspacing="0" cellpadding="0" width="100%"&gt;&lt;tr valign="top"&gt;&lt;td style="WIDTH: 27pt" align="right"&gt;&lt;div&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Wingdings"&gt;&amp;#167;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div style="MARGIN-LEFT: 0pt; TEXT-INDENT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;&lt;font style="DISPLAY: inline; FONT-SIZE: 9pt; FONT-FAMILY: Helvetica LT Std Light, sans-serif"&gt;Cleco Power owns and operates Acadia Unit 1 and operates Acadia Unit 2 on behalf of Acadia or a future owner.&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;</NonNumbericText><NonNumericTextHeader>Note 19 &amp;#8212; Acadia Unit 1 Transaction  On February 23, 2010, Cleco Power completed the acquisition of Acadia Unit 1 and half of Acadia Power</NonNumericTextHeader><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>us-types:textBlockItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>Disclosure of all information related to any significant acquisition and disposal. Disclosure may include methodology and assumptions, type of asset, asset classification, useful life, useful purpose, acquisition cost, method of acquisition or disposal, depreciation method, gain or loss on disposal pretax and net of tax, date of acquisition or disposal and restrictions on amount of proceeds from donated assets.</ElementDefenition><ElementReferences>No authoritative reference available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Acadia Unit 1 Transaction</Label></Row></Rows><Footnotes /><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows><ReportName>Acadia Unit 1 Transaction</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>false</HasCustomUnits><SharesShouldBeRounded>true</SharesShouldBeRounded></InstanceReport>
