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Liabilities, Net Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Extended payment terms period Supply Chain Financing, Extended Payment Terms Period Supply Chain Financing, Extended Payment Terms Period Fluence v.s. Diablo Energy Storage Fluence v.s. Diablo Energy Storage [Member] Fluence v.s. Diablo Energy Storage Entity Small Business Entity Small Business Total comprehensive (loss) income attributable to Fluence Energy, Inc. 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Finite-Lived, before Accumulated Amortization Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] Liquidated Damages Liquidated Damages [Abstract] Liquidated Damages Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Non-Controlling Interest Consolidation, Subsidiaries or Other Investments, Consolidated Entities, Policy [Policy Text Block] Executive Category: Executive Category [Axis] Long-Term Purchase Commitment [Table] Long-Term Purchase Commitment [Table] Service requirement (in years) Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period Counterparty Name [Axis] Counterparty Name [Axis] Goodwill [Roll Forward] Goodwill [Roll Forward] Liabilities and Stockholders’ Equity Liabilities and Equity [Abstract] Other Commitments [Table] Other Commitments [Table] Intangible Asset, Finite-Lived, Class [Domain] Intangible Asset, Finite-Lived, Class [Domain] Leverage ratio Debt Instrument, Covenant, Net Leverage Ratio, Maximum Threshold Debt Instrument, Covenant, Net Leverage Ratio, Maximum Threshold Outstanding obligations Supplier Finance Program, Obligation Inventory, net Net Inventory, Net Award Type [Domain] Award Type [Domain] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Letter of Credit Issued Letter of Credit Issued [Member] Letter of Credit Issued Warranty balance, beginning Warranty balance, ending Standard and Extended Product Warranty Accrual Supplemental Cash Flows Information Supplemental Cash Flow Information [Abstract] Letters of credit under bilateral credit facilities Letters of Credit Under the Bilateral Credit Facility [Member] Letters of Credit Under the Bilateral Credit Facility Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Non-Controlling Interest Ownership (AES) Subsidiary, Ownership Percentage, Noncontrolling Owner PEO Name PEO Name Debt instrument, threshold percentage of stock price trigger Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger Geographical [Axis] Geographical [Axis] AES Grid Stability AES Grid Stability, LLC [Member] AES Grid Stability, LLC Accruals and provisions Increase (Decrease) in Accrued Liabilities Total restricted cash Restricted Cash Deferred revenue, beginning of period Deferred revenue end of period Contract with Customer, Liability Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] Guarantees issued Supply Chain Financing Guarantee By Related Party Supply Chain Financing Guarantee By Related Party Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Entity Tax Identification Number Total Interest Expense, Debt Convertible senior notes, net Convertible Notes Payable, Noncurrent Other Commitments [Line Items] Other Commitments [Line Items] Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Collateral for surety program Surety Program Collateral [Member] Surety Program Collateral Cash, cash equivalents, and restricted cash as of the beginning of the period Cash, cash equivalents, and restricted cash as of the end of the period Total cash, cash equivalents and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Schedule of Finite-Lived Intangible Assets [Table] Intangible Asset, Finite-Lived [Table] Accounts payable Increase (Decrease) in Accounts Payable Siemens Industry Siemens Industry, Inc. [Member] Siemens Industry, Inc. Accruals Other Accrued Liabilities, Current Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Base Rate Base Rate [Member] Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Non-NEOs Non-NEOs [Member] Performance period Share-Based Compensation Arrangement By Share-Based Payment Award, Cumulative Adjusted EBITDA During Performance Period Share-Based Compensation Arrangement By Share-Based Payment Award, Cumulative Adjusted EBITDA During Performance Period Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Total comprehensive (loss) income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Debt Instrument [Axis] Debt Instrument [Axis] Stock-based compensation Share-Based Payment Arrangement, Noncash Expense Capitalized software to be sold, Accumulated Amortization Capitalized Cost, Software to be Sold, Leased, or Marketed, Accumulated Amortization Payment terms, threshold days Supplier Finance Program, Payment Terms Classification, Threshold Days Supplier Finance Program, Payment Terms Classification, Threshold Days Schedule of Net Carrying Value and Fair Value of the Convertible Senior Notes Schedule of Debt [Table Text Block] Effect of AES Redemption of Class B-1 common stock for Class A common stock (in shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Customer [Domain] Customer [Domain] Schedule of Inventory, Current Schedule of Inventory, Current [Table Text Block] Leases Lessee, Operating Leases [Text Block] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Basic (in usd per share) Net (loss) income attributable to Fluence Energy, Inc. - basic (in usd per share) Earnings Per Share, Basic Promissory Note Promissory Note [Member] Promissory Note Deferred revenue Contract with Customer, Liability, Current Total current assets Assets, Current Adjust Term SOFR Rate, Adjusted EURIBOR Rate, Or Adjusted Term CORRA Rate Adjust Term SOFR Rate, Adjusted EURIBOR Rate, Or Adjusted Term CORRA Rate [Member] Adjust Term SOFR Rate, Adjusted EURIBOR Rate, Or Adjusted Term CORRA Rate Statistical Measurement [Axis] Statistical Measurement [Axis] Insider Trading Arrangements [Line Items] Supplier Finance Program [Table] Supplier Finance Program [Table] Award Type [Axis] Award Type [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Contractual period Revenue From Contract With Customer, Contractual Period Revenue From Contract With Customer, Contractual Period Trade receivables, net Accounts Receivable, after Allowance for Credit Loss, Current Fluence Energy, Inc Fluence Energy, INC [Member] Fluence Energy, INC Maximum Exposure Range for Each Agreement Contractual Obligation, Exposure Per Agreement Contractual Obligation, Exposure Per Agreement Accumulated deficit Retained Earnings (Accumulated Deficit) Product and Service [Axis] Product and Service [Axis] Debt instrument, threshold trading days Debt Instrument, Convertible, Threshold Trading Days Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Common Stock Common Stock [Member] Effect of remeasurement of non-controlling interest due to other share transactions Noncontrolling Interest, Decrease from Remeasurement Noncontrolling Interest, Decrease from Remeasurement Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Summary of Significant Accounting Policies and Estimates Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Cover [Abstract] Outstanding non-qualified stock options Non-Qualified Stock Options [Member] Non-Qualified Stock Options Coupon interest Interest Expense, Debt, Excluding Amortization IEEPA Tariff Refund IEEPA Tariff Refund [Member] IEEPA Tariff Refund Shares underlying the conversion option in the 2030 Convertible Senior Notes Shares Underlying The Conversion Option In The 2030 Convertible Senior Notes [Member] Shares Underlying The Conversion Option In The 2030 Convertible Senior Notes Derivative Contract [Domain] Derivative Contract [Domain] Repayment installments Debt Instrument, Number Of Repayment Installments Debt Instrument, Number Of Repayment Installments Total revenue percentage Concentration Risk, Percentage Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Foreign currency adjustment Goodwill, Foreign Currency Translation, Gain (Loss) Spare parts Spare Parts [Member] Spare Parts Equity Awards Adjustments Equity Awards Adjustments [Member] 2026 Purchase Obligation, to be Paid, Remainder of Fiscal Year Non Qualified Stock Option Non Qualified Stock Option [Member] Non Qualified Stock Option Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Other Non-Current Liabilities Derivative Liability, Noncurrent Total liabilities and stockholders’ equity Liabilities and Equity Outstanding pre-IPO options issued pursuant to the 2020 Unit Option Plan Share-Based Payment Arrangement, Option [Member] Non-Controlling interests Equity, Attributable to Noncontrolling Interest Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Plan Name [Axis] Plan Name [Axis] Related Party [Domain] Related and Nonrelated Parties [Domain] Number of suppliers Number of Suppliers Participating in Program Number of Suppliers Participating in Program Inventory Disclosure [Abstract] Tabular List, Table Tabular List [Table Text Block] Vesting [Axis] Vesting [Axis] Additional paid-in capital Additional Paid in Capital Collateral for credit card program Credit Card Program Collateral [Member] Credit Card Program Collateral Other non-current liabilities Increase (Decrease) in Other Noncurrent Liabilities Other income, net Other Nonoperating Income (Expense) Taxes recoverable, tax credits and tariff refunds Income Taxes Receivable, Current Name Awards Close in Time to MNPI Disclosures, Individual Name Cumulative adjusted EBITDA during performance period, percentage Share-Based Compensation Arrangement By Share-Based Payment Award, Performance Metric Weight Percentage, Adjusted EBITDA Share-Based Compensation Arrangement By Share-Based Payment Award, Performance Metric Weight Percentage, Adjusted EBITDA Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Forfeited (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeited in Period Non-current assets: Assets, Noncurrent [Abstract] General and administrative General and administrative General and Administrative Expense Movement in Standard and Extended Product Warranty Accrual, Increase (Decrease) [Roll Forward] Movement in Standard and Extended Product Warranty Accrual, Increase (Decrease) [Roll Forward] Fluence Energy, LLC Fluence Energy, LLC [Member] Fluence Energy, LLC Sale of stock, price (in usd per share) Sale of Stock, Price Per Share Purchases under supply chain financing arrangements Purchases Under Supplier Financing Arrangements Purchases Under Supplier Financing Arrangements Effect of AES Redemption of Class B-1 common stock for Class A common stock Stock Issued During Period, Value, Conversion of Convertible Securities Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Total receivables from related parties Receivables, Net, Current Preferred stock, $0.00001 per share, 10,000,000 shares authorized; no shares issued and outstanding as of June 30, 2026 and September 30, 2025 Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] Threshold performance, percentage Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Rights, Below-Threshold Performance, Percentage Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Rights, Below-Threshold Performance, Percentage Amount reclassified from accumulated other comprehensive income (loss) into earnings Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Class of Stock [Domain] Class of 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Licensing Agreements [Member] Patents And Licensing Agreements Name Measure Name Business Combination, Consideration Transferred, Equity Interest, Type [Domain] Business Combination, Consideration Transferred, Equity Interest, Type [Domain] Total right-of-use assets Lease, Right-of-Use Asset Lease, Right-of-Use Asset Income Taxes Income Tax Disclosure [Text Block] Schedule of Cash and Cash Equivalents [Table] Cash and Cash Equivalent [Table] Accounts payable Accounts Payable, Current Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Debt Instrument [Line Items] Debt Instrument [Line Items] Other current assets Increase (Decrease) in Other Current Assets Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Integrated systems, batteries, and other equipment Integrated Systems, Batteries, And Other Equipment [Member] Integrated Systems, Batteries, And Other Equipment Additional Paid-In Capital Additional Paid-in Capital [Member] Entity Filer Category Entity Filer Category Impairment of goodwill Goodwill, Impairment Loss Total Accrued Liabilities Subsidiary or Equity Method Investee, Sale of Stock [Table] Subsidiary or Equity Method Investee, Sale of Stock [Table] Schedule of Assets and Liabilities, Operating Lease Assets and Liabilities, Lessee [Table Text Block] Assets and Liabilities, Lessee Name Trading Arrangement, Individual Name Deferred revenue Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Cash and cash equivalents Cash and Cash Equivalent 2021 Plan 2021 Incentive Award Plan [Member] 2021 Incentive Award Plan Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Capital expenditures on software and other Payments for Software Class B-2 common stock Common Class B-2 [Member] Common Class B-2 Company Selected Measure Amount Company Selected Measure Amount Consulting Services Revenue Consulting Services Revenue [Member] Consulting Services Revenue Number of classes of common stock Number Of Classes Of Common Stock Number Of Classes Of Common Stock Depreciation and amortization Depreciation, Depletion and Amortization 2027 Other Commitment, to be Paid, Year One Cost Inventory, Gross Entity Address, Address Line Two Entity Address, Address Line Two Stock-based compensation (in shares) Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture Net (loss) income Net (loss) income Net loss Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Class B-1 common stock Class B-1 common stock Common Class B-1 [Member] Common Class B-1 Current assets: Assets, Current [Abstract] Debt Conversion, Name [Domain] Debt Conversion, Name [Domain] Non-PEO NEO Non-PEO NEO [Member] Entity Address, Address Line One Entity Address, Address Line One Additions Contract with Customer, Liability, Additions Contract with Customer, Liability, Additions Income tax expense (benefit) Income Tax Expense (Benefit) Customer Concentration Risk Customer Concentration Risk [Member] Cost of goods and services Cost of goods and services Cost of Product and Service Sold Total Purchase Obligation Award Timing Method Award Timing Method [Text Block] Consolidation Consolidation, Policy [Policy Text Block] Net decrease in cash, cash equivalents, and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Provision for receivables from related parties Receivables, Related Parties, Allowance For Credit Loss Receivables, Related Parties, Allowance For Credit Loss Sale of Receivables Transfers and Servicing of Financial Assets [Text Block] 30 month anniversary of IPO Share-Based Payment Arrangement, Tranche Three [Member] Net (loss) income attributable to Fluence Energy, Inc. Net Income (Loss) Attributable to Parent Net Intangible Asset, Finite-Lived, and Capitalized Cost, Software to be Sold, Leased, or Marketed, after Accumulated Amortization Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Statement [Table] Statement [Table] 18 month anniversary of IPO Share-Based Payment Arrangement, Tranche Two [Member] Restatement Determination Date: Restatement Determination Date [Axis] Proceeds from exercise of stock options Proceeds from Stock Options Exercised Net proceeds sale of receivable Transfer of Financial Assets Accounted for as Sales, Cash Proceeds Received for Assets Derecognized, Amount Tariff refund claims submitted IEEPA Tariff Refund Claims Submitted IEEPA Tariff Refund Claims Submitted Collateral for outstanding bank guarantees Outstanding Bank Guarantee Collateral [Member] Outstanding Bank Guarantee Collateral Leases Lessee, Leases [Policy Text Block] Schedule of Change in Contract Liability Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] AOCI Attributable To Parent Before Reclassification And Tax [Roll Forward] AOCI Attributable To Parent Before Reclassification And Tax [Roll Forward] AOCI Attributable To Parent Before Reclassification And Tax Debt instrument, threshold consecutive trading days Debt Instrument, Convertible, Threshold Consecutive Trading Days Damages sought, value Loss Contingency, Damages Sought, Value Category of Item Purchased [Axis] Category of Item Purchased [Axis] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Derivative Instruments and Hedging Activities Disclosure [Abstract] Accruals and Provisions Long-Term Contracts or Programs Disclosure [Text Block]