Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2016
Summary of Significant Accounting Policies
Principles of Consolidation	Principles of Consolidation The accompanying unaudited interim consolidated financial statements include the financial statements of the Company and its wholly-owned subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation.
Interim Financial Statements	Interim Financial Statements The accompanying unaudited interim consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (SEC). In the opinion of management, all adjustments, which consist solely of normal recurring adjustments, necessary to present fairly, in accordance with U.S. generally accepted accounting principles (GAAP), the financial position, results of operations and cash flows for all periods presented, have been made. The results of operations for the interim periods presented are not necessarily indicative of the results that may be expected for the full year. Certain information and footnote disclosures normally included in annual consolidated financial statements prepared in accordance with GAAP have been condensed or omitted. These unaudited interim consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K, filed for the fiscal year ended December 31, 2015. The information presented in the accompanying consolidated balance sheet as of December 31, 2015 has been derived from the Company’s December 31, 2015 audited consolidated financial statements. All other information has been derived from the unaudited interim consolidated financial statements of the Company.
Revenue Recognition	Revenue Recognition The Company recognizes revenue under arrangements for products and services, which may include the sale of products and related services, including revenue from installation, service and maintenance, spare parts, hydrogen fueling services (which may include hydrogen supply as well as hydrogen fueling infrastructure) and leased units. The Company also recognizes revenue under research and development contracts, which are primarily cost reimbursement contracts associated with the development of PEM fuel cell technology. Products and Services The Company enters into revenue arrangements that may contain a combination of fuel cell systems and equipment, installation, service, maintenance, spare parts, and other support services. Revenue arrangements containing fuel cell systems and equipment may be sold, or leased to customers. For these multiple deliverable arrangements, the Company accounts for each separate deliverable as a separate unit of accounting if the delivered item or items have value to the customer on a standalone basis. The Company considers a deliverable to have standalone value if the item is sold separately by us or another entity or if the item could be resold by the customer. The Company allocates revenue to each separate deliverable based on its relative selling price. For a majority of our deliverables, the Company determines relative selling prices using its best estimate of the selling price since vendor-specific objective evidence and third-party evidence is generally not available for the deliverables involved in its revenue arrangements due to a lack of a competitive environment in selling fuel cell technology. When determining estimated selling prices, the Company considers the cost to produce the deliverable, a reasonable gross margin on that deliverable, the selling price and profit margin for similar products and services, the Company’s ongoing pricing strategy and policies, the value of any enhancements that have been built into the deliverable and the characteristics of the varying markets in which the deliverable is sold, as applicable. The Company determines estimated selling prices for deliverables in its arrangements based on the specific facts and circumstances of each arrangement and analyzes the estimated selling prices used for its allocation of consideration of each arrangement. Once relative selling prices are determined, the Company proportionately allocates the sale consideration to each element of the arrangement. The allocated sales consideration related to fuel cell systems and equipment, spare parts, and hydrogen infrastructure is recognized as revenue at shipment if title and risk of loss have passed to the customer, there is persuasive evidence of an arrangement, the sales price is fixed or determinable, collection of the related receivable is reasonably assured, and customer acceptance criteria, if any, have been successfully demonstrated. The allocated sales consideration related to service and maintenance is generally recognized as revenue on a straight-line basis over the term of the contract, as appropriate. With respect to sales of consigned spare parts, the Company does not recognize revenue until the risks and rewards of ownership have transferred, the price is fixed, and the Company has a reasonable expectation of collection upon billing. For those customers who do not purchase an extended maintenance contract, 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. Only a limited number of fuel cell units are under standard warranty. In a vast majority of its commercial transactions, the Company sells extended maintenance contracts that generally provide for a five to ten year warranty from the date of product installation. These types of contracts are accounted for as a separate deliverable, and accordingly, revenue generated from these transactions is deferred and recognized in income over the warranty period, 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 in income 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 on the life of the contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience, contractual agreements and the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. The Company is a party to PPAs with certain key customers, such as Walmart. Revenue associated with these agreements are treated as rental income and recognized on a straight-line basis over the life of the agreements. The Company also has rental expense associated with sale/leaseback agreements with financial institutions that were entered into commensurate with the PPAs. Rental expense is also recognized on a straight-line basis over the life of the agreements and is characterized as cost of PPA revenue on the accompanying unaudited interim consolidated statement of operations. The Company purchases hydrogen fuel from suppliers and sells to its customers upon delivery. Revenue and cost of revenue related to this fuel is recorded as dispensed and delivered, respectively, and are included in the respective “Fuel delivered to customers” lines on the unaudited interim consolidated statements of operations. Research and Development Contracts Contract accounting is used for research and development contract revenue. The Company generally shares in the cost of these programs with cost sharing percentages ranging from 30% to 50% of total project costs. Revenue from time and material contracts is recognized on the basis of hours expended plus other reimbursable contract costs incurred during the period and is included within the “other” revenue line on the unaudited interim consolidated statement of operations. All allowable work performed through the end of each calendar quarter is billed, subject to limitations in the respective contracts. We expect to continue research and development contract work that is directly related to our current product development efforts.
Sale-leaseback transactions	Sale/leaseback transactions The Company provides its products and services to certain customers in the form of a PPA that generally expire in six years. For these specific transactions, the Company will complete a sale/leaseback for the related assets to a financial institution for similar terms. The Company accounts for sale/leaseback transactions as operating leases in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Subtopic 840-40, Leases – Sale/Leaseback Transactions.
Cash Equivalents	Cash Equivalents For purposes of the unaudited interim 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. At March 31, 2016 and December 31, 2015, cash equivalents consist of money market accounts. The Company’s cash and cash equivalents are deposited with financial institutions primarily located in the U.S. and may at times exceed insured limits.
Common Stock Warrant Accounting	Common Stock Warrant Accounting The Company accounts for common stock warrants in accordance with applicable accounting guidance provided in ASC Subtopic 815-40, Derivatives and Hedging – Contracts in Entity’s Own Equity, as either derivative liabilities or as equity instruments depending on the specific terms of the warrant agreement. In compliance with applicable securities law, registered common stock warrants that require the issuance of registered shares upon exercise and do not sufficiently preclude an implied right to cash settlement are accounted for as derivative liabilities. We currently classify these derivative warrant liabilities on the accompanying unaudited interim consolidated balance sheets as a long-term liability, which is revalued at each balance sheet date subsequent to the initial issuance using the Black-Scholes pricing model. The Black-Scholes pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in the fair value of the warrants are reflected in the accompanying unaudited interim consolidated statements of operations as change in fair value of common stock warrant liability.
Use of Estimates	Use of Estimates The unaudited interim 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 unaudited interim consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reclassifications	Reclassifications Reclassifications are made, whenever necessary, to prior period financial statements to conform to the current period presentation. These reclassifications did not have a net impact the results of operations or net cash flows in the periods presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In March 2016, an accounting update was issued to simplify various aspects related to how share-based payments are accounted for and presented. This accounting update is effective for annual periods beginning after December 15, 2016 and interim periods within those years. Early adoption is permitted in any interim or annual period, with any adjustments reflected as of the beginning of the fiscal year of adoption. The Company is evaluating the impact this update will have on the consolidated financial statements. In February 2016, an accounting update was issued which requires balance sheet recognition for operating leases, among other changes to previous lease guidance. This accounting update is effective for fiscal years beginning after December 15, 2018. The Company is evaluating the impact this update will have on the consolidated financial statements. In July 2015, an accounting update was issued that changes inventory measurement from lower of cost or market to lower of cost and net realizable value. The new standard applies to inventory measured at first-in, first-out (FIFO). This accounting update is effective for the annual reporting periods beginning after December 15, 2016, and interim periods within those years. The Company does not expect the adoption of this update to have a significant effect on the consolidated financial statements. In August 2014, an accounting update was issued relating to how management assesses conditions and events that could raise substantial doubt about an entity’s ability to continue as a going concern. This accounting update is effective for reporting periods ending after December 15, 2016, and for annual and interim periods thereafter. The Company does not expect the adoption of this update to have a significant effect on the consolidated financial statements. In June 2014, an accounting update was issued that replaces the existing revenue recognition framework regarding contracts with customers. In July 2015, the Financial Accounting Standards Board (FASB) announced a one year delay in the required adoption date from January 1, 2017 to January 1, 2018. The Company is evaluating the effect this update will have on the consolidated financial statements and has not yet selected a transition method.