<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>00000025 - Disclosure - NOTES PAYABLE - RELATED PARTIES</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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</LabelSeparator><Level>1</Level><ElementName>XPWR_NotesToFinancialStatementsAbstract</ElementName><ElementPrefix>XPWR_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Notes to Financial Statements</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_RelatedPartyTransactionsDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="From2013-03-01to2013-05-31" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="font: 10pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;Notes
payable &amp;#150; related parties consists of two separate notes:&lt;/font&gt;&lt;/p&gt;

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&lt;table cellspacing="0" cellpadding="0" align="center" style="width: 80%; border-collapse: collapse; font-size: 10pt"&gt;
&lt;tr style="vertical-align: bottom"&gt;
    &lt;td&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td colspan="3" style="border-bottom: black 1pt solid; text-align: center"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;May
    31, 2013&lt;/font&gt;&lt;/td&gt;
    &lt;td style="padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td colspan="3" style="border-bottom: black 1pt solid; text-align: center"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;February
    28, 2013&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: rgb(204,238,204); vertical-align: bottom"&gt;
    &lt;td style="text-align: justify; padding-left: 5.4pt; width: 69%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;Purchase
    order (PO) financing note payable due within five days of receipt by Company, in whole or in part, portion of funds collected
    on collateral sales order, or, Company may submit a new collateral sales order with value equal to or in excess of principal
    outstanding. Interest rate is 1% per month. The note maturity date was originally May 15, 2013. On April 1, 2013, under a
    new simple note agreement, the maturity date was extended until October 14, 2013 at 10% interest. Under the new terms, interest
    is reduced to 10%, and monthly payments of $4,937 are due May 1, 2013 until October 1, 2014, when all remaining principal
    and interest are due. Under the note, outstanding accrued interest was added to principal as of the amendment date. The May
    31, 2013 balance is shown net of a $46,062 debt discount.&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; width: 1%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right; width: 13%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;473,938&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; width: 1%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="width: 1%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; width: 1%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right; width: 13%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;520,000&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; width: 1%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: white; vertical-align: bottom"&gt;
    &lt;td style="padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: rgb(204,238,204); vertical-align: bottom"&gt;
    &lt;td style="text-align: justify; padding-bottom: 1pt; padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;Promissory
    note bearing a 10% annual interest rate. Unsecured. The note maturity date was originally May 1, 2013. On April 1, 2013, under
    a new simple note agreement, the maturity date was extended until October 14, 2013 at 10% interest. Under the note, outstanding
    accrued interest was added to principal as of the amendment date. The May 31, 2013 balance is shown net of a $46,554 debt
    discount.&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;123,446&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;170.000&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: white; vertical-align: bottom"&gt;
    &lt;td style="text-align: right; padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;Totals&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;597,384&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;690,000&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: rgb(204,238,204); vertical-align: bottom"&gt;
    &lt;td style="text-align: right; padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: white; vertical-align: bottom"&gt;
    &lt;td style="text-align: right; padding-bottom: 1pt; padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;Less:
    current maturities&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;0&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 1pt solid; text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;0&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: rgb(204,238,204); vertical-align: bottom"&gt;
    &lt;td style="text-align: right; padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="background-color: white; vertical-align: bottom"&gt;
    &lt;td style="text-align: right; padding-bottom: 2.5pt; padding-left: 5.4pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;Long-term
    portion&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.5pt double; text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.5pt double; text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;597,384&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; padding-bottom: 2.5pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="padding-bottom: 2.5pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.5pt double; text-align: left"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.5pt double; text-align: right"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;690,000&lt;/font&gt;&lt;/td&gt;
    &lt;td style="text-align: left; padding-bottom: 2.5pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif"&gt;&amp;#160;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
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