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Summary of Significant Accounting Policies | 12 Months Ended
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Dec. 31, 2023
Summary of Significant Accounting Policies |
Summary of Significant Accounting Policies | 2. Summary of Significant Accounting Policies ​ Principles of Consolidation ​ The consolidated financial statements include the financial statements of the Company and its wholly-owned subsidiaries. Intercompany balances and transactions have been eliminated in consolidation. In addition, we include our share of the results of our joint ventures HyVia, AccionaPlug S.L. and SK Plug Hyverse, and our investment in Clean H2 Infra Fund, using the equity method based on our economic ownership interest and our ability to exercise significant influence over the operating and financial decisions of HyVia, AccionaPlug S.L., SK Plug Hyverse and Clean H2 Infra Fund. ​ Use of Estimates ​ The consolidated financial statements of the Company have been prepared in conformity with U.S. generally accepted accounting principles, which require management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. On an on-going basis, we evaluate our estimates and judgments, including but not limited to those related to revenue recognition, valuation of inventories, goodwill and intangible assets, valuation of long-lived assets, accrual for service loss contracts, operating and finance leases, allowance for doubtful accounts receivable, unbilled revenue, common stock warrants, stock-based compensation, income taxes, and contingencies. We base our estimates and judgments on historical experience and on various other factors and assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about (1) the carrying values of assets and liabilities and (2) the amount of revenue and expenses realized that are not readily apparent from other sources. Actual results may differ from these estimates under different assumptions or conditions. ​ Leases ​ The Company is a lessee in noncancelable (1) operating leases, primarily related to sale/leaseback transactions with financial institutions for deployment of the Company’s products at certain customer sites, and (2) finance leases. The Company accounts for leases in accordance with Accounting Standards Codification (ASC) Topic 842, Leases (ASC Topic 842), as amended. ​ The Company determines if an arrangement is or contains a lease at contract inception. The Company recognizes a right of use asset and a lease liability at the lease commencement date. For operating leases, the lease liability is initially measured at the present value of the unpaid lease payments at the lease commencement date. For finance leases, the lease liability is initially measured in the same manner and date as for operating leases and is subsequently measured at amortized cost using the effective interest method. ​ Key estimates and judgments include how the Company determines (1) the discount rate it uses to discount the unpaid lease payments to present value, (2) the lease term, and (3) the lease payments. ​
| ● | ASC Topic 842 requires a lessee to discount its future lease payments using the interest rate implicit in the lease or, if that rate cannot be readily determined, its incremental borrowing rate. Generally, the Company cannot determine the interest rate implicit in its lease because it does not have access to the lessor’s estimated residual value or the amount of the lessor’s deferred initial direct costs. Therefore, the Company generally uses its incremental borrowing rate to estimate the discount rate for each lease. The Company’s incremental borrowing rate for a lease is the rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. Adjustments that considered the Company’s actual borrowing rate, inclusive of securitization, as well as borrowing rates for companies of similar credit quality, were applied in the determination of the incremental borrowing rate.
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| ● | The lease term for all of the Company’s leases includes the noncancelable period of the lease, plus any additional periods covered by either a Company option to extend (or not to terminate) the lease that the Company is reasonably certain to exercise, or an option to extend (or not to terminate) the lease controlled by the lessor.
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| ● | Lease payments included in the measurement of the lease liability comprise fixed payments, and for certain finance leases, the exercise price of a Company option to purchase the underlying asset if the Company is reasonably certain at lease commencement to exercise the option.
​ The right of use asset is initially measured at cost, which comprises the initial amount of the lease liability adjusted for lease payments made at or before the lease commencement date, plus any initial direct costs incurred less any lease incentives received. For operating leases, the right of use asset is subsequently measured throughout the lease term at the carrying amount of the lease liability, plus initial direct costs, plus (minus) any prepaid (accrued) lease payments, less the unamortized balance of lease incentives received. Lease expense for lease payments is recognized on a straight-line basis over the lease term. ​ For finance leases, the right of use asset is subsequently amortized using the straight-line method from the lease commencement date to the earlier of the end of the useful life of the underlying asset or the end of the lease term unless the lease transfers ownership of the underlying asset to the Company or the Company is reasonably certain to exercise an option to purchase the underlying asset. In those cases, the right of use asset is amortized over the useful life of the underlying asset. Amortization of the right of use asset is recognized and presented separately from interest expense on the lease liability. The Company’s leases do not contain variable lease payments. ​ Right of use assets for operating and finance leases are periodically reviewed for impairment losses. The Company uses the long-lived assets impairment guidance in ASC Subtopic 360-10, Property, Plant and Equipment — Overall, to determine whether a right of use asset is impaired, and if so, the amount of the impairment loss to recognize. ​ The Company monitors for events or changes in circumstances that require a reassessment of its leases. When a reassessment results in the remeasurement of a lease liability, a corresponding adjustment is made to the carrying amount of the corresponding right of use asset. ​ Operating and finance lease right of use assets are presented separately on the Company’s consolidated balance sheets. The current portions of operating and finance lease liabilities are also presented separately within current liabilities and the long-term portions are presented separately within noncurrent liabilities on the consolidated balance sheets. ​ The Company has elected not to recognize right of use assets and lease liabilities for short-term leases that have a lease term of 12 months or less. The Company recognizes the lease payments associated with its short-term leases as an expense on a straight-line basis over the lease term. ​ Revenue Recognition ​ The Company enters into contracts that may contain one or a combination of fuel cell systems and infrastructure, installation, maintenance, spare parts, fuel delivery and other support services. Contracts containing fuel cell systems and related infrastructure may be sold directly to customers or provided to customers under a PPA. The Company also enters into contracts that contain electrolyzer stacks, systems, maintenance, and other support services. Furthermore, the Company enters into contracts related to the sales of cryogenic equipment, liquefaction systems and engineered equipment. ​ The Company does not include a right of return on its products other than rights related to standard warranty provisions that permit repair or replacement of defective goods. The Company accrues for anticipated standard warranty costs at the same time that revenue is recognized for the related product, or when circumstances indicate that warranty costs will be incurred, as applicable. Any prepaid amounts would only be refunded to the extent services have not been provided or the fuel cell systems or infrastructure have not been delivered. ​ Revenue is measured based on the transaction price specified in a contract with a customer, subject to the allocation of the transaction price to distinct performance obligations as discussed below. The Company recognizes revenue when it satisfies a performance obligation by transferring a product or service to a customer. ​ Promises to the customer are separated into performance obligations and are accounted for separately if they are (1) capable of being distinct and (2) distinct in the context of the contract. The Company considers a performance obligation to be distinct if the customer can benefit from the good or service either on its own or together with other resources readily available to the customer and the Company’s promise to transfer the goods or service to the customer is separately identifiable from other promises in the contract. The Company allocates revenue to each distinct performance obligation based on relative standalone selling prices. ​ Payment terms for sales of fuel cells, infrastructure, and service to customers are typically 30 to 90 days from shipment of the goods. Payment terms on electrolyzer systems are typically based on achievement of milestones over the term of the contract with the customer. Sale/leaseback transactions with financial institutions are invoiced and collected upon transaction closing. Service is prepaid upfront in a majority of the arrangements. The Company does not adjust the transaction price for a significant financing component when the performance obligation is expected to be fulfilled within a year. ​ The Company has issued to each of Amazon.com NV Investment Holdings LLC and Walmart warrants to purchase shares of the Company’s common stock. The Company presents the provision for common stock warrants within each revenue-related line item on the consolidated statements of operations. This presentation reflects the discount that those common stock warrants represent, and therefore revenue is net of these non-cash charges. The provision of common stock warrants is allocated to the relevant revenue-related line items based upon the expected mix of the revenue for each respective contract. See Note 17, “Warrant Transaction Agreements”, for more details. ​ Nature of goods and services ​ The following is a description of principal activities from which the Company generates its revenue. ​
| (b) | Sales of equipment, related infrastructure and other
​ (j)Sales of fuel cell systems, related infrastructure and equipment ​ Revenue from sales of fuel cell systems, related infrastructure, and equipment represents sales of our GenDrive units, GenSure stationary backup power units, as well as hydrogen fueling infrastructure. ​ The Company uses a variety of information sources in determining standalone selling prices for fuel cells systems and the related infrastructure. For GenDrive fuel cells, given the nascent nature of the Company’s market, the Company considers several inputs, including prices from a limited number of standalone sales as well as the Company’s negotiations with customers. The Company also considers its costs to produce fuel cells as well as comparable list prices in estimating standalone selling prices. The Company uses applicable observable evidence from similar products in the market to determine standalone selling prices for GenSure stationary backup power units and hydrogen fueling infrastructure. The determination of standalone selling prices of the Company’s performance obligations requires significant judgment, including periodic assessment of pricing approaches and available observable evidence in the market. Once relative standalone selling prices are determined, the Company proportionately allocates the transaction price to each performance obligation within the customer arrangement based upon standalone selling price. The allocated transaction price related to fuel cell systems and spare parts is recognized as revenue at a point in time which usually occurs upon delivery (and occasionally at time of shipment). Revenue on hydrogen infrastructure installations is generally recognized at the point at which transfer of control passes to the customer, which usually occurs upon customer acceptance of the hydrogen infrastructure. The Company uses an input method to determine the amount of revenue to recognize during each reporting period when such revenue is recognized over time, based on the costs incurred to satisfy the performance obligation. ​ (ii) Sales of electrolyzer systems and solutions ​ Revenue from sales of electrolyzer systems and solutions represents sales of electrolyzer stacks and systems used to generate hydrogen for various applications including mobility, ammonia production, methanol production, power to gas, and other uses. ​ The Company uses a variety of information sources in determining standalone selling prices for electrolyzer systems solutions. Electrolyzer stacks are typically sold on a standalone basis and the standalone selling price is the contractual price with the customer. The Company uses an adjusted market assessment approach to determine the standalone selling price of electrolyzer systems when sold with extended service or other equipment. This includes considering both standalone selling prices of the systems by the Company and available information on competitor pricing on similar products. The determination of standalone selling prices of the Company’s performance obligations requires judgment, including periodic assessment of pricing approaches and available observable evidence in the market. Once relative standalone selling prices are determined, the Company proportionately allocates the transaction price to each performance obligation within the customer arrangement based upon standalone selling price. Revenue on electrolyzer systems and stacks is generally recognized at the point at which transfer of control passes to the customer, which usually occurs upon title transfer at shipment or delivery to the customer location. In certain instances, control of electrolyzer systems transfers to the customer over time, and the related revenue is recognized over time as the performance obligation is satisfied. We recognize revenue over time when contract performance results in the creation of a product for which we do not have an alternative use and the contract includes an enforceable right to payment in an amount that corresponds directly with the value of the performance completed. In these instances, we use an input measure (cost-to-total cost or percentage-of-completion method) of progress to determine the amount of revenue to recognize during each reporting period based on the costs incurred to satisfy the performance obligation. ​ Payments received from customers are recorded within deferred revenue and customer deposits in the consolidated balance sheets until control is transferred. The related cost of such product and installation is also deferred as a component of deferred cost of revenue in the consolidated balance sheets until control is transferred. ​ (iii) Sales of cryogenic equipment and other ​ Revenue from sales of cryogenic equipment represents sales of liquefaction system and other cryogenic equipment such as trailers and mobile storage equipment for the distribution of liquefied hydrogen, oxygen, argon, nitrogen and other cryogenic gases. ​ The Company uses a variety of information sources in determining standalone selling prices for liquefaction systems and cryogenic equipment. Liquefaction systems are typically sold on a standalone basis and the standalone selling price is the contractual price with the customer. The Company uses an adjusted market assessment approach to determine the standalone selling price of liquefaction systems when sold with other equipment. This includes considering both standalone selling prices of the systems by the Company and available information on competitor pricing on similar products. The determination of standalone selling prices of the Company’s performance obligation requires judgment, including periodic assessment of pricing approaches and available observable evidence in the market. Revenue on liquefaction systems is generally recognized over time. Control transfers to the customer over time, and the related revenue is recognized over time as the performance obligation is satisfied. We recognize revenue over time when contract performance results in the creation of a product for which we don’t not have an alternative use and the contract includes an enforceable right to payment in an amount that corresponds directly with the value of the performance completed. In these instances, we use an input measure of progress to determine the amount of revenue to recognize during each reporting period based on the costs incurred to satisfy the performance obligation. ​ Revenue on cryogenic equipment is generally recognized at the point at which transfer of control passes to the customer, which usually occurs upon title transfer at shipment or delivery to the customer location. ​ Payments received from customers are recorded within deferred revenue and customer deposits in the consolidated balance sheets until control is transferred. The related costs of such product and installation is also deferred as a component of deferred cost of revenue in the consolidated balance sheets until control is transferred. ​ (b) Services performed on fuel cell systems and related infrastructure ​ Revenue from services performed on fuel cell systems and related infrastructure represents revenue earned on our service and maintenance contracts and sales of spare parts. The Company uses an adjusted market assessment approach to determine standalone selling prices for services. This approach considers market conditions and constraints while maximizing the use of available observable inputs obtained from a limited number of historical standalone service renewal prices and negotiations with customers. The transaction price allocated to services as discussed above is generally recognized as revenue over time on a straight-line basis over the expected service period, as customers simultaneously receive and consume the benefits of routine, recurring maintenance performed throughout the contract period. ​ In substantially all of its transactions, the Company sells extended maintenance contracts that generally provide for a five-to-ten-year service period from the date of product installation in exchange for an up-front payment. Services include monitoring, technical support, maintenance and related services. These services are accounted for as a separate performance obligation, and accordingly, revenue generated from these transactions, subject to the proportional allocation of transaction price, is deferred and recognized as revenue over the term of the contract, generally on a straight-line basis. Additionally, the Company may enter into annual service and extended maintenance contracts that are billed monthly. Revenue generated from these transactions is recognized as revenue on a straight-line basis over the term of the contract. Costs are recognized as incurred over the term of the contract. When costs are projected to exceed revenues over the life of the extended maintenance contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives, if any. The actual results may differ from these estimates. See “Extended Maintenance Contracts” below. ​ Extended maintenance contracts generally do not contain customer renewal options. Upon expiration, customers may either negotiate a contract extension or switch to purchasing spare parts and maintaining the fuel cell systems on their own. ​
| (c) | Power purchase agreements
​ Revenue from PPAs primarily represents payments received from customers who make monthly payments to access the Company’s GenKey solution. ​ Revenue associated with these agreements is recognized on a straight-line basis over the life of the agreements as the customers simultaneously receive and consume the benefits from the Company’s performance of the services. The customers receive services ratably over the contract term. ​ In conjunction with entering into a PPA with a customer, the Company may enter into a separate transaction with third-party financial institutions in which the Company receives proceeds from the sale/leaseback transactions of the equipment and the sale of future service revenue. The proceeds from the financial institution are allocated between the sale of equipment and the sale of future service revenue based on the relative standalone selling prices of equipment and service. The proceeds allocated to the sale of future services are recognized as finance obligations. The proceeds allocated to the sale of the equipment are evaluated to determine if the transaction meets the criteria for sale/leaseback accounting. To meet the sale/leaseback criteria, control of the equipment must transfer to the financial institution, which requires among other criteria the leaseback to meet the criteria for an operating lease and the Company must not have a right to repurchase the equipment (unless specific criteria are met). These transactions typically meet the criteria for sale/leaseback accounting and accordingly, the Company recognizes revenue on the sale of the equipment, and separately recognizes the leaseback obligations. ​ The Company recognizes an operating lease liability for the equipment leaseback obligation based on the present value of the future payments to the financial institutions that are attributed to the equipment leaseback. The discount rate used to determine the lease liability is the Company’s incremental borrowing rate. The Company also records a right of use asset which is amortized over the term of the leaseback. Rental expense is recognized on a straight-line basis over the life of the leaseback and is included as a cost of power purchase agreements revenue on the consolidated statements of operations. ​ Certain of the Company’s transactions with financial institutions do not meet the criteria for sale/leaseback accounting and accordingly, no equipment sale is recognized. All proceeds from these transactions are accounted for as finance obligations. The right of use assets related to these transactions are classified as equipment related to the PPAs and fuel delivered to the customers, net in the consolidated balance sheets. The Company uses its transaction-date incremental borrowing rate as the interest rate for its finance obligations that arise from these transactions. No additional adjustments to the incremental borrowing rate have been deemed necessary for the finance obligations that have resulted from the failed sale/leaseback transactions. ​ In determining whether the sales of fuel cells and other equipment to financial institutions meet the requirements for revenue recognition under sale/leaseback accounting, the Company, as lessee, determines the classification of the lease. The Company estimates certain key inputs to the associated calculations such as: 1) discount rate used to determine the present value of future lease payments, 2) fair value of the fuel cells and equipment, and 3) useful life of the underlying asset(s): ​
| ● | ASC Topic 842 requires a lessee to discount its future lease payments using the interest rate implicit in the lease or, if that rate cannot be readily determined, its incremental borrowing rate. Generally, the Company cannot determine the interest rate implicit in its leases because it does not have access to the lessor’s estimated residual value or the amount of the lessor’s deferred initial direct costs. Therefore, the Company generally uses its incremental borrowing rate to estimate the discount rate for each lease. Adjustments that considered the Company’s actual borrowing rate, inclusive of securitization, as well as borrowing rates for companies of similar credit quality were applied in the determination of the incremental borrowing rate.
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| ● | In order for the lease to be classified as an operating lease, the present value of the future lease payments cannot exceed 90% of the fair value of the leased assets. The Company estimates the fair value of the lease assets using the sales prices.
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| ● | In order for a lease to be classified as an operating lease, the lease term cannot exceed 75% (major part) of the estimated useful life of the leased asset. The average estimated useful life of the fuel cells is 10 years, and the average estimated useful life of the hydrogen infrastructure is 20 years. These estimated useful lives are compared to the term of each lease to determine the appropriate lease classification.
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| (d) | Fuel delivered to customers and related equipment
​ Revenue associated with fuel delivered to customers represents the sale of hydrogen to customers that has been purchased by the Company from a third party or generated at our hydrogen production plant. The stand-alone selling price is not estimated because it is sold separately and therefore directly observable. ​ The Company purchases hydrogen fuel from suppliers in most cases (and sometimes produces hydrogen onsite) and sells to its customers. Revenue and cost of revenue related to this fuel is recorded as dispensed and is included in the respective fuel delivered to customers and related equipment lines on the consolidated statements of operations. ​
| (e) | Other revenue
​ Other revenue includes payments received for technical services that include engineering services, program management services, procurement services and operations, testing and validation services with HyVia. The scope of these services includes mutually agreed upon services as may be requested from time to time by HyVia. Other revenue also includes sales of electrolyzer engineering and design services. The scope of these services includes establishing and defining project technical requirements, standards and guidelines as well as assistance in scoping and scheduling of large-scale electrolyzer solutions. ​ Contract costs ​ The Company expects that incremental commission fees paid to employees as a result of obtaining sales contracts are recoverable and therefore the Company capitalizes them as contract costs. ​ Capitalized commission fees are amortized on a straight-line basis over the period of time which the transfer of goods or services to which the assets relate occur, typically ranging from five to ten years. Amortization of the capitalized commission fees is included in selling, general and administrative expenses. ​ The Company recognizes the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that the Company otherwise would have recognized is one year or less. These costs are included in selling, general and administrative expenses. Capitalized contract costs at December 31, 2023 and 2022 were $0.8 million and $0.6 million, respectively. ​ Cash and cash equivalents ​ For purposes of the consolidated statements of cash flows, the Company considers all highly-liquid debt instruments with original maturities of three months or less to be cash equivalents. Due to their short-term nature, the carrying amounts reported in the consolidated balance sheets approximate the fair value of cash and cash equivalents. The Company’s cash and cash equivalents are deposited with financial institutions located in the U.S. and may at times exceed insured limits. ​ Restricted cash ​ Restricted cash consists primarily of cash that serves as support for leasing arrangements. Any cash that is legally restricted from use is classified as restricted cash. If the purpose of restricted cash relates to acquiring a long-term asset, liquidating a long-term liability, or is otherwise unavailable for a period longer than one year from the balance sheet date, the restricted cash is included in other long-term assets. Otherwise, restricted cash is included in other current assets in the consolidated balance sheets. ​ Available-for-sale securities ​ Available-for-sale securities is comprised of U.S. Treasury securities, certificates of deposit and corporate bonds, with original maturities greater than three months. We consider these securities to be available for use in our current operations, and therefore classify them as current even if we do not dispose of the securities in the following year. ​ Available-for-sale securities are recorded at fair value as of each balance sheet date. As of each balance sheet date, unrealized gains and losses, with the exception of credit related losses, are recorded to accumulated other comprehensive income/(loss). Any credit related losses are recognized as a credit loss allowance on the balance sheet with a corresponding adjustment to the statement of operations. Realized gains and losses are due to the sale and maturity of securities classified as available-for-sale and includes the loss from accumulated other comprehensive loss reclassifications for previously unrealized losses on available-for-sale debt securities. ​ As of December 31, 2023, the Company has no investments classified as available-for-sale. ​ Equity securities ​ Equity securities are comprised of fixed income and equity market index mutual funds. Equity securities are valued at fair value with changes in the fair value recognized in our consolidated statements of operations. We consider these securities to be available for use in our current year operations, and therefore classify them as current even if we do not dispose of the securities in the following year. ​ As of December 31, 2023, the Company has no investments classified as equity securities. ​ Investments in non-consolidated entities and non-marketable equity securities ​ The Company accounts for its investments in non-consolidated entities, such as HyVia, AccionaPlug S.L., SK Plug Hyverse and Clean H2 Infra Fund, as equity method investments. ​ Included in “Investments in non-consolidated entities and non-marketable equity securities” on the consolidated balance sheet are equity investments without readily determinable fair values (“non-marketable equity securities”). Non-marketable equity securities that do not qualify for equity method accounting are measured at cost, less any impairment and adjusted for changes resulting from observable price changes in orderly transactions for the identical or similar investments of the same issuer. Our investment in non-marketable equity securities was $5.6 million and $8.8 million as of December 31, 2023 and 2022, respectively. ​ The Company sells goods and services to related parties, including its equity method investees, which are conducted at arm’s length in the normal course of business. Transactions involving services do not result in assets remaining on the books of the investee, and therefore no profit elimination is recorded in accordance with ASC Subtopic 323-10-35, Equity Method and Joint Ventures. Transactions involving inventory are evaluated if the assets remain on the books of the investee or if they have been sold to a third party – intra-entity profits are eliminated for transactions in which assets remain on the books of the investee. ​ Common stock warrant accounting ​ The Company accounts for common stock warrants as either derivative liabilities or as equity instruments depending on the specific terms of the respective warrant agreements. Common stock warrants that meet certain applicable requirements of ASC Subtopic 815-40, Derivatives and Hedging – Contracts in Entity’s Own Equity, and other related guidance, including the ability of the Company to settle the warrants without the issuance of registered shares or the absence of rights of the grantee to require cash settlement, are accounted for as equity instruments. The Company classifies these equity instruments within additional paid-in capital on the consolidated balance sheets. ​ Common stock warrants accounted for as equity instruments represent the warrants issued to Amazon and Walmart as discussed in Note 17, “Warrant Transaction Agreements”. The Company adopted FASB ASU 2019-08, Compensation – Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606), which requires entities to measure and classify share-based payment awards granted to a customer. ​ In order to calculate warrant charges, the Company used the Black-Scholes pricing model, which required key inputs including volatility and risk-free interest rate and certain unobservable inputs for which there is little or no market data, requiring the Company to develop its own assumptions. The Company estimated the fair value of unvested warrants, considered to be probable of vesting, at the time. Based on that estimated fair value, the Company determined warrant charges, which are recorded as a reduction of revenue in the consolidated statement of operations. ​ Accounts receivable ​ Accounts receivable are stated at the amount billed or billable to customers and are ordinarily due between 30 and 90 days after the issuance of the invoice. Receivables are reserved or written off based on individual credit evaluation and specific circumstances of the customer. The allowance for expected credit losses for current accounts receivable is based primarily on past collections experience relative to the length of time receivables are past due; however, when available evidence reasonably supports an assumption that counterparty credit risk over the expected payment period will differ from current and historical payment collections, a forecasting adjustment will be reflected in the allowance for expected credit losses. The allowance for doubtful accounts and related receivable are reduced when the amount is deemed uncollectible. As of December 31, 2023, and 2022, the allowance for doubtful accounts was $8.8 million and $43 thousand, respectively. ​ Inventory ​ Inventories are valued at the lower of cost, determined on a first-in, first-out basis, and net realizable value. All inventory, including spare parts inventory held at service locations, is not relieved until the customer has received the product, at which time the customer obtains control of the goods. We maintain inventory levels adequate for our short-term needs within the next twelve months based upon present levels of production. An allowance for potential non-saleable inventory due to damaged, excess stock or obsolescence is based upon a detailed review of inventory, past history, and expected usage. The Company's estimate of the reserves utilizes certain inputs and involves judgment. The Company evaluates excess and obsolescence and lower of cost or net realizable value inventory reserves on a quarterly basis and, as necessary, reserves inventory based upon a variety of factors, including historical usage, forecasted usage and sales, product obsolescence, anticipated selling price, and anticipated cost to complete to determine product margin and other factors. We review all contracts related to product lines with projected negative margins that are arranged to be sold at a loss in the future as the basis for a lower of cost or net realizable value adjustment. ​ Property, plant and equipment ​ Property, plant and equipment are originally recorded at cost or, if acquired as part of a business combination, at fair value. Maintenance and repairs are expensed as costs are incurred. Depreciation on plant and equipment is calculated on the straight-line method over the estimated useful lives of the assets. Gains and losses resulting from the sale of property and equipment are recorded in current operations. Included within machinery and equipment is certain equipment related to our hydrogen plants. The Company records depreciation and amortization over the following estimated useful lives: ​
Building and leasehold improvements |  | 10 – 30 years
Machinery and equipment | ​ | 2 – 30 years
Software | ​ | 1 – 5 years
Hydrogen production plants | ​ | 30 years
​ Equipment related to PPAs and fuel delivered to customers ​ Equipment related to PPAs and fuel delivered to customers primarily consists of the assets deployed related to PPAs and sites where we deliver fuel to customers as well as equipment related to failed sale/leaseback transactions. Equipment is depreciated over its useful life. Depreciation expense is recorded on a straight-line basis and is included in cost of revenue for PPAs or cost of fuel delivered to customers, respectively, in the consolidated statements of operations. ​ Impairment ​ Contract assets ​ During the fourth quarter of 2023, there was a contract asset impairment charge of $2.4 million related to our assessment of recoverability of a customer contract. There was no such impairment charge for the year ended December 31, 2022. ​ Other current assets ​ During the second quarter of 2023, there was an other current asset impairment charge of $9.7 million related to the termination of a commercial agreement. There was no such impairment charge for the year ended December 31, 2022. ​ Property, equipment, leasehold improvements, and finite-lived intangible assets ​ Long-lived assets, such as property, equipment, leasehold improvements, and finite-lived intangible assets, are reviewed for impairment whenever events and circumstances indicate that the carrying amount of an asset may not be recoverable. If circumstances require a long-lived asset or asset group to be tested for impairment, the Company first compares undiscounted cash flows expected to be generated by that asset or asset group to its carrying amount. If the carrying amount of the long-lived asset or asset group is not recoverable on an undiscounted cash flow basis, an impairment is recognized to the extent that the carrying amount exceeds its fair value. In making these determinations, the Company uses certain assumptions, including, but not limited to: (i) estimated fair value of the assets; and (ii) estimated, undiscounted future cash flows expected to be generated by these assets, which are based on additional assumptions such as asset utilization, length of service, the asset will be used in the Company’s operations, and (iii) estimated residual values. Fair value is determined using various valuation techniques including discounted cash flow models, quoted market values and third-party independent appraisals, as considered necessary. ​ There was impairment of $3.1 million and $0.8 million of property, equipment, leasehold improvements, or finite-lived intangible assets during the years ended December 31, 2023 and 2022, respectively. ​ PPA Executory Contract Considerations ​ We evaluate PPA assets on a quarterly basis to identify events or changes in circumstances (“triggering events”) that indicate the carrying value of certain assets may not be recoverable. PPA assets that we evaluate include right of use lease assets, equipment deployed to our PPAs, and assets related primarily to our fuel delivery business. ​ Upon the occurrence of a triggering event, PPA assets are evaluated on a per-site basis to determine if the carrying amounts are recoverable. The determination of recoverability is made based upon the estimated undiscounted future net cash flows of assets grouped at the lowest level for which there are identifiable cash flows independent of the cash flows of other groups. For operating assets, the Company has generally determined that the lowest level of identifiable cash flows is based on the customer sites. The assets related primarily to our fuel delivery business are considered to be their own asset group. The cash flows are estimated based on the remaining useful life of the primary asset within the asset group. ​ For assets related to our PPA agreements, we consider all underlying cash inflows related to our contract revenues and cash outflows relating to the costs incurred to service the PPAs. Our cash flow estimates used in the recoverability test, are based upon, among other things, historical results adjusted to reflect our best estimate of future cash flows and operating performance. Development of future cash flows also requires us to make assumptions and to apply judgment, including timing of future expected cash flows, future cost savings initiatives, and determining recovery values. Changes to our key assumptions related to future performance and other economic and market factors could adversely affect the outcome of our recoverability tests and cause more asset groups to be tested for impairment. ​ If the estimated undiscounted future net cash flows for a given asset group are less than the carrying amount of the related asset group, an impairment loss is determined by comparing the estimated fair value with the carrying amount of the asset group. The impairment loss is then allocated to the assets in the asset group based on the asset’s relative carrying amounts. However, assets are not impaired below their then estimated fair values. Fair value is generally determined through various valuation techniques, including discounted cash flow models, quoted market values and third-party independent appraisals, as well as year-over-year trends in pricing of our new equipment and overall evaluation of our industry and market, as considered necessary. The Company considers these indicators with certain of its own internal indices and metrics in determining fair value in light of the nascent state of the Company’s market and industry. The estimate of fair value represents our best estimates of these factors and is subject to variability. Changes to our key assumptions related to future performance and other economic and market factors could adversely affect our impairment evaluation. ​ In 2023, the Company has determined that the assets deployed for certain PPA arrangements, as well as certain assets related to the delivery of fuel to customers, are not recoverable based on the undiscounted estimated future cash flows of the asset group, and an expense of $4.8 million was recorded to impairment on the income statement. As the PPA arrangements are considered to be executory contracts and there is no specific accounting guidance that permits loss recognition for these revenue contracts, the Company has not recognized a provision for the expected future losses under these revenue arrangements. The Company expects that it will recognize future service losses for these arrangements as it continues its efforts to reduce costs of delivering the maintenance component of these arrangements. The Company has estimated total future revenues and costs for these types of arrangements based on existing contracts and leverage of the related assets. For the future estimates, the Company used service cost estimates for extended maintenance contracts and customer warrant provisions at rates consistent with experience to date. The terms for the underlying estimates vary but the average residual term on the existing contracts is four years. ​ Intangible assets ​ Intangible assets consist of acquired technology, customer relationships, trade name and other finite intangibles and are amortized using a straight-line method over their useful lives. Additionally, the intangible assets are reviewed for impairment when certain triggering events occur. ​ Extended maintenance contracts ​ On a quarterly basis, we evaluate any potential losses related to our extended maintenance contracts for fuel cell systems and related infrastructure that has been sold. We measure loss accruals at the customer contract level. The expected revenues and expenses for these contracts include all applicable expected costs of providing services over the remaining term of the contracts and the related unearned net revenue. A loss is recognized if the sum of expected costs of providing services under the contract exceeds related unearned net revenue and is recorded as a provision for loss contracts related to service in the consolidated statements of operations. A key component of these estimates is the expected future service costs. In estimating the expected future service costs, the Company considers its current service cost level and applies judgement related to certain cost saving estimates that have been implemented in the field. The expected future cost savings will be primarily dependent upon the success of the Company’s initiatives related to increasing stack life and achieving better economies of scale on service labor. If the expected cost saving initiatives are not realized, this will increase the costs of providing services and could adversely affect our estimated contract loss accrual. Further, as we continue to work to improve quality and reliability; however, unanticipated additional quality issues or warranty claims may arise and additional material charges may be incurred in the future. These quality issues could also adversely affect our contract loss accrual. The Company has undertaken and will soon undertake several other initiatives to extend the life and improve the reliability of its equipment. As a result of these initiatives and our additional expectation that the increase in certain costs will abate, the Company believes that its contract loss accrual is sufficient. However, if elevated service costs persist, the Company will adjust its estimated future service costs and increase its contract loss accrual estimate. ​ The following table shows the roll forward of balances in the accrual for loss contracts, including changes due to the provision for loss accrual, releases to service cost of sales, provision for warrants and foreign currency translation adjustment (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​
​ | Year ended | ​ | Year ended
​ | December 31, 2023 |  | December 31, 2022
Beginning balance | $ | 81,066 | ​ | $ | 89,773
Provision for loss accrual | ​ | 85,375 | ​ | ​ | 23,295
Releases to service cost of sales | ​ | (29,713) | ​ | ​ | (35,446)
Increase to loss accrual related to customer warrants | ​ | 971 | ​ | ​ | 3,506
Foreign currency translation adjustment | ​ | 154 | ​ | ​ | (62)
Ending balance | $ | 137,853 | ​ | $ | 81,066
​ The Company increased its provision for loss accrual to $137.9 million for the year ended December 31, 2023 due to continued cost and inflationary increases of labor, parts and related overhead coupled with the timing of the remaining period of service required. As a result, the Company increased its estimated projected costs to service existing fuel cell systems and the related infrastructure. ​ Goodwill ​ Goodwill is an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. The Company reviews goodwill for impairment at least annually. ​ In accordance with ASC 350, Intangibles — Goodwill and Other, we test goodwill for impairment on an annual basis on October 31 and between annual tests if indicators of potential impairment exist. The impairment test compares the fair value of the reporting units to their carrying amounts to assess whether impairment exists. We have reviewed the provisions of ASC 350-20 with respect to the criteria necessary to evaluate the number of reporting units that exist. Based on this review, we have concluded that we have one operating segment and one reporting unit. During the annual impairment review process, the Company has the option to perform a qualitative assessment over relevant events and circumstances to determine whether it is more-likely-than-not that the fair value of the reporting unit is less than its carrying amount or to perform a quantitative assessment. We derive the fair value of our reporting unit using the market approach, which estimates fair value based on the determination of comparable publicly-traded companies and market multiples of revenue and earnings derived from those companies with similar operating and investment characteristics as the reporting unit being valued. The Company compares and reconciles the fair value of the reporting unit to our market capitalization in order to assess the reasonableness of the calculated fair value by reporting unit. If the carrying value of the reporting unit exceeds its fair value, an impairment loss equal to the excess is recorded. ​ The Company performs an impairment review of goodwill on an annual basis at October 31, and when a triggering event is determined to have occurred between annual impairment tests. The Company’s stock price declined below book value during the fourth quarter of 2023. Management believes the decline of the stock price was due primarily to missed projections and reduced liquidity. Based on the results of our annual review, the Company recognized an impairment charge of $249.5 million for the year ended December 31, 2023. The Company’s analyses did not indicate impairment of goodwill for the years ended December 31, 2022 and 2021. See Note 10, “Intangible Assets and Goodwill”, for further information. ​ Fair value measurements ​ The Company records the fair value of assets and liabilities in accordance with ASC 820, Fair Value Measurement (“ASC 820”). ASC 820 defines fair value as the price received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date and in the principal or most advantageous market for that asset or liability. The fair value should be calculated based on assumptions that market participants would use in pricing the asset or liability, not on assumptions specific to the entity. ​ In addition to defining fair value, ASC 820 expands the disclosure requirements around fair value and establishes a fair value hierarchy for valuation inputs. The hierarchy prioritizes the inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market. Each fair value measurement is reported in one of the three levels, which is determined by the lowest level input that is significant to the fair value measurement in its entirety. ​ These levels are: ​
| ● | Level 1 — quoted prices (unadjusted) in active markets for identical assets or liabilities.
| ● | Level 2 — quoted prices for similar assets and liabilities in active markets or inputs that are observable for the asset or liability, either directly or indirectly through market corroboration, for substantially the full term of the financial instrument.
| ● | Level 3 — unobservable inputs reflecting management’s own assumptions about the inputs used in pricing the asset or liability at fair value.
​ Income taxes ​ Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. A valuation allowance is recorded to reduce the carrying amounts of deferred tax assets if it is more likely than not that such assets will not be realized. ​ The Company accounts for uncertain tax positions in accordance with FASB ASC No. 740-10-25, Income Taxes-Overall-Recognition. The Company recognizes in its consolidated financial statements the impact of a tax position only if that position is more likely than not to be sustained on audit, based on the technical merits of the position. The Company recognizes interest and penalties related to unrecognized tax benefits on the interest expense line and other expense, net line, respectively, in the accompanying consolidated statements of operations. ​ Foreign currency translation ​ Foreign currency translation adjustments arising from conversion of the Company’s foreign subsidiary’s financial statements to U.S. dollars for reporting purposes are included in accumulated other comprehensive income/(loss) in stockholders’ equity on the consolidated balance sheet. Transaction gains and losses resulting from the effect of exchange rate changes on transactions denominated in currencies other than the functional currency of the Company’s operations give rise to realized foreign currency transaction gains and losses and are included in other expense, net in the consolidated statements of operations. ​ Research and development ​ Activities that qualify as research and development under ASC 730 include: (i) laboratory research aimed at discovery of new knowledge; (ii) searching for applications of new research findings or other knowledge; (iii) conceptual formulation and design of possible product or process alternatives; (iv) testing in search for or evaluation of product or process alternatives; (v) modification of the formulation or design of a product or process: (vi) design, construction, and testing of preproduction prototypes and models; (vii) design of tools, jigs, molds, and dies involving new technology; (viii) design, construction, and operation of a pilot plant that is not of a scale economically feasible to the entity for commercial production; (ix) engineering activity required to advance the design of a product to the point that it meets specific functional and economic requirements and is ready for manufacture; and (x) design and development of tools used to facilitate research and development or components of a product or process that are undergoing research and development activities. Costs related to research and development activities by the Company are expensed as incurred. ​ Stock-based compensation ​ The Company maintains employee stock-based compensation plans, which are described more fully in Note 19, “Employee Benefit Plans”. Stock-based compensation represents the cost related to stock-based awards granted to employees and directors. The Company measures stock-based compensation cost at grant-date, based on the fair value of the award, and recognizes the cost as expense on a straight-line basis over the option’s requisite service period. Forfeitures are recognized as they occur. ​ The Company estimates the fair value of stock-based awards using a Black-Scholes valuation model. Stock-based compensation expense is recorded in cost of revenue associated with sales of fuel cell systems, related infrastructure and equipment, cost of revenue for services performed on fuel cell systems and related infrastructure, research and development expense and selling, general and administrative expenses in the consolidated statements of operations based on the employees’ respective function. ​ Beginning in September 2021, the Company also issued performance stock option awards that include a market condition. The grant date fair value of performance stock options is estimated using a Monte Carlo simulation model and the cost is recognized using the accelerated attribution method. ​ The Company records deferred tax assets for awards that result in deductions on the Company’s income tax returns, based upon the amount of compensation cost recognized and the Company’s statutory tax rate. Differences between the deferred tax assets recognized for financial reporting purposes and the actual tax deduction reported on the Company’s income tax return are recorded in the income statement. ​ Convertible senior notes ​ The Company accounts for its convertible senior notes as a single liability measured at amortized cost. The Company uses the effective interest rate method to amortize the debt issuance costs to interest expense over the respective term of the convertible senior notes. ​ Subsequent Events ​ The Company evaluates subsequent events at the date of the balance sheet as well as conditions that arise after the balance sheet date but before the consolidated financial statements are issued. The effects of conditions that existed at the balance sheet date are recognized in the consolidated financial statements. Events and conditions arising after the balance sheet date but before the consolidated financial statements are issued are evaluated to determine if disclosure is required to keep the consolidated financial statements from being misleading. To the extent such events and conditions exist, if any, disclosures are made regarding the nature of events and the estimated financial effects for those events and conditions. See Note 24, “Subsequent Events”. ​ Recent Accounting Pronouncements ​ Recently Issued and Not Yet Adopted Accounting Pronouncements ​ In March 2020, ASU 2020-03, Codification Improvements to Financial Instruments, was issued to make various codification improvements to financial instruments to make the standards easier to understand and apply by eliminating inconsistencies and providing clarifications. This update will be effective at various dates beginning with date of issuance of this ASU. The adoption of this standard will not have a material impact on the Company’s consolidated financial statements. ​ In November 2023, ASU 2023-07, Improvements to Reportable Segment Disclosures, was issued to improve reportable segment disclosure requirements, primarily through enhanced disclosures about significant segment expenses. The purpose of the amendments is to enable investors to better understand an entity’s overall performance and assess potential future cash flows. This update will be effective for fiscal years beginning after December 15, 2023. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements. ​ In December 2023, ASU 2023-09, Improvements to Income Tax Disclosures, was issued to require public business entities to annually disclose specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. Additionally, annual disclosures on income taxes paid will be required to be further disaggregated by federal, state, and foreign taxes. This update will be effective for annual periods beginning after December 15, 2024. The adoption of this standard will not have a material impact to our consolidated financial statements. However, we are currently evaluating the impact of this ASU on our income tax disclosures.

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