Document and Entity Information Document Information [Table] Document Information [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common Class A [Member] Class A common stock Class A Common Class B-1 Common Class B-1 [Member] Class B-1 common stock Class B-1 common stock Document Information [Line Items] Document and Entity Information Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Two Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag CONDENSED CONSOLIDATED BALANCE SHEETS Statement [Table] Statement [Table] Common Class B-2 Common Class B-2 [Member] Class B-2 common stock Related and Nonrelated Parties [Axis] Related Party [Axis] Related and Nonrelated Parties [Domain] Related Party [Domain] Nonrelated Party [Member] Nonrelated Party Related Party [Member] Related Party Statement [Line Items] Statement Assets [Abstract] Right-of-use assets: Assets Assets, Current [Abstract] Current assets: Cash and Cash Equivalent Cash and cash equivalents Restricted Cash, Current Total restricted cash Restricted cash Accounts Receivable, after Allowance for Credit Loss, Current Trade receivables, net Contract with Customer, Asset, after Allowance for Credit Loss, Current Unbilled receivables Other Receivables, Net, Current Receivables from related parties Accounts receivable Advances on Inventory Purchases Advances to suppliers Advance payment Inventory, Net. Inventory, net Net Other Assets, Current Total Other current assets Assets, Current Total current assets Assets, Noncurrent [Abstract] Non-current assets: Property, Plant and Equipment, Net Property and equipment, net Intangible Assets, Net (Excluding Goodwill) Intangible assets, net Goodwill. Goodwill, End of the period Goodwill, Beginning of the period Goodwill Deferred Income Tax Assets, Net Deferred income tax asset Other Assets, Noncurrent Other non-current assets Assets, Noncurrent Total non-current assets Assets Total assets Liabilities and Equity [Abstract] Liabilities and Stockholders' Equity Liabilities, Current [Abstract] Current liabilities: Accounts Payable, Current Accounts payable Contract with Customer, Liability, Current Deferred revenue Employee-related Liabilities, Current Personnel related liabilities Accrued Liabilities, Current Accruals and provisions Accruals and provisions Taxes Payable, Current Taxes payable Other Liabilities, Current Other current liabilities Liabilities, Current Total current liabilities Liabilities, Noncurrent [Abstract] Non-current liabilities: Deferred Income Tax Liabilities, Net Deferred income tax liability Convertible Notes Payable, Noncurrent Convertible senior notes, net Other Liabilities, Noncurrent Other non-current liabilities Liabilities, Noncurrent Total non-current liabilities Liabilities Total liabilities Equity [Abstract] Stockholders' Equity: Preferred Stock, Value, Issued Preferred stock, $0.00001 per share, 10,000,000 shares authorized; no shares issued and outstanding as of March 31, 2026 and September 30, 2025 Common Stock, Value, Issued Common stock Treasury Stock, Value Treasury stock, at cost Additional Paid in Capital Additional paid-in capital Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated other comprehensive income Retained Earnings (Accumulated Deficit) Accumulated deficit Equity, Attributable to Parent Total stockholders' equity attributable to Fluence Energy, Inc. Equity, Attributable to Noncontrolling Interest Non-Controlling interests Equity, Including Portion Attributable to Noncontrolling Interest Ending balance Beginning balance Total stockholders' equity Liabilities and Equity Total liabilities and stockholders' equity Preferred Stock, Par or Stated Value Per Share Preferred stock, par value (in usd per share) Preferred Stock, Shares Authorized Preferred stock, shares authorized (in shares) Preferred Stock, Shares Issued Preferred stock, shares issued (in shares) Preferred Stock, Shares Outstanding Preferred stock, shares outstanding (in shares) Common Stock, Par or Stated Value Per Share Common stock, par value (in usd per share) Common Stock, Shares Authorized Common stock, shares authorized (in shares) Common Stock, Shares, Issued Common stock, shares issued (in shares) Common Stock, Shares, Outstanding Ending balance (in shares) Beginning balance (in shares) Common stock, shares outstanding (in shares) CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Revenue from Contract with Customer, Excluding Assessed Tax Total revenue Revenue Cost of Product and Service Sold Cost of goods and services Cost of goods and services Gross Profit Gross profit Operating Expenses [Abstract] Operating expenses: Research and Development Expense Research and development Research and development expenses Selling and Marketing Expense Sales and marketing General and Administrative Expense General and administrative General and administrative expenses Depreciation, Depletion and Amortization, Nonproduction Depreciation and amortization Interest Income (Expense), Nonoperating Interest expense (income), net Other Nonoperating Income (Expense) Other (income) expense, net Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Loss before income taxes Income Tax Expense (Benefit) Income tax expense (benefit) Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net loss Net loss Net loss Net Income (Loss) Attributable to Noncontrolling Interest Net loss attributable to non-controlling interest Net loss attributable to Fluence Energy, Inc. Weighted Average Number of Shares Outstanding, Basic [Abstract] Weighted average number of Class A common shares outstanding: Weighted Average Number of Shares Outstanding, Basic Basic (in shares) Weighted Average Number of Shares Outstanding, Diluted Diluted (in shares) Loss per Share Loss per share of Class A common stock: Earnings Per Share, Basic Basic (in usd per share) Net loss attributable to Fluence Energy, Inc., $ per share - basic (in usd per share) Earnings Per Share, Diluted Diluted (in usd per share) Net loss attributable to Fluence Energy, Inc., $ per share - diluted (in usd per share) CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax (Loss) gain on foreign currency translation, net of tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax (Loss) gain on cash flow hedges, net of tax (Loss) gain on cash flow hedges, net of tax Other Comprehensive Income (Loss), Net of Tax Total other comprehensive (loss) income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Total comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive loss attributable to non-controlling interest Comprehensive Income (Loss), Net of Tax, Attributable to Parent Total comprehensive loss attributable to Fluence Energy, Inc. CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock [Member] Common Stock Additional Paid-in Capital [Member] Additional Paid-In Capital Retained Earnings [Member] Accumulated Deficit AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) Treasury Stock, Common [Member] Treasury Stock Noncontrolling Interest [Member] Non-Controlling interest Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity Treasury Stock, Common, Shares Ending balance (in shares) Beginning balance (in shares) Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture Stock-based compensation (in shares) APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Stock-based compensation Share-Based Payment Arrangement, Shares Withheld for Tax Withholding Obligation Class A common stock withheld related to settlement of employee taxes for stock-based compensation awards (in shares) Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation Class A common stock withheld related to settlement of employee taxes for stock-based compensation awards Noncontrolling Interest, Decrease from Remeasurement Noncontrolling Interest, Decrease from Remeasurement Effect of remeasurement of non-controlling interest due to other share transactions Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period Proceeds from exercise of stock options (in shares) Stock Issued During Period, Value, Stock Options Exercised Proceeds from exercise of stock options Adjustments To Additional Paid In Capital, Option Indexed To Issuer's Equity Adjustments To Additional Paid In Capital, Option Indexed To Issuer's Equity Purchases of Capped Calls related to 2030 Convertible Senior Notes Premium paid Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Distribution to AES Grid Stability CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS Cash Provided by (Used in) Operating Activity, Continuing Operation [Abstract] Operating activities Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Adjustments to reconcile net loss to net cash used in operating activities: Depreciation, Depletion and Amortization Depreciation and amortization Amortization of Debt Issuance Costs Amortization of debt issuance costs Inventory Write-down Inventory provision Share-Based Payment Arrangement, Noncash Expense Stock-based compensation Deferred Income Tax Expense (Benefit) Deferred income taxes Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Changes in operating assets and liabilities: Increase (Decrease) in Accounts Receivable Trade receivables, net Increase (Decrease) in Contract with Customer, Asset Unbilled receivables Increase (Decrease) in Due from Related Parties, Current Receivables from related parties Increase (Decrease) to Advances to Inventory Purchases Increase (Decrease) to Advances on Inventory Purchases Advances to suppliers Increase (Decrease) in Inventories Inventory Increase (Decrease) in Other Current Assets Other current assets Increase (Decrease) in Other Noncurrent Assets Other non-current assets Increase (Decrease) in Accounts Payable Accounts payable Increase (Decrease) in Contract with Customer, Liability Deferred revenue Increase (Decrease) in Accrued Liabilities Accruals and provisions Increase (Decrease) in Income Taxes Payable Taxes payable Increase (Decrease) in Other Current Liabilities Other current liabilities Increase (Decrease) in Other Noncurrent Liabilities Other non-current liabilities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Net cash used in operating activities Cash Provided by (Used in) Investing Activity, Continuing Operation [Abstract] Investing activities Payments for Software Capital expenditures on software and other Payments to Acquire Property, Plant, and Equipment Purchase of property and equipment Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Net cash used in investing activities Cash Provided by (Used in) Financing Activity, Continuing Operation [Abstract] Financing activities Payment, Tax Withholding, Share-Based Payment Arrangement Class A common stock withheld related to settlement of employee taxes for stock-based compensation awards Proceeds from Convertible Debt Proceeds from issuance of 2030 Convertible Senior Notes Payments Of Capped Calls Related To Convertible Notes Payments Of Capped Calls Related To Convertible Notes Purchases of Capped Calls related to 2030 Convertible Senior Notes Payments of Merger Related Costs, Financing Activities Payment for debt issuance costs Proceeds from Short-Term Debt Purchases under supply chain financing arrangements Proceeds from Stock Options Exercised Proceeds from exercise of stock options Payments of Ordinary Dividends, Noncontrolling Interest Distribution to AES Grid Stability Finance Lease, Principal Payments Principal payments on finance leases Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Net cash provided by financing activities Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Effect of exchange rate changes on cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Net (decrease) increase in cash, cash equivalents, and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Cash, cash equivalents, and restricted cash as of the end of the period Cash, cash equivalents, and restricted cash as of the beginning of the period Total cash, cash equivalents and restricted cash Supplemental Cash Flow Information [Abstract] Supplemental Cash Flows Information Interest Paid, Excluding Capitalized Interest, Operating Activity Interest paid Income Taxes Paid, Net Cash paid on income taxes Organization and Operations Nature of Operations [Text Block] Organization and Operations Summary of Significant Accounting Policies and Estimates Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Summary of Significant Accounting Policies and Estimates Earnings Per Share [Text Block] Loss per Share Revenue from Contracts with Customers Revenue from Contract with Customer [Text Block] Revenue from Contracts with Customers