5. Intangible Assets

        Intangible assets consisted of the following (in thousands):

        Amortization expense for the intangible assets was $0.8 million, $1.1 million, and $0.3 million for the years ended March 31, 2012, 2011 and 2010.

        Expected future amortization expense of intangible assets as of March 31, 2012 is as follows:

        On February 1, 2010, the Company acquired the TA100 microturbine generator product line (the "MPL") from CPS to expand the Company's microturbine product line and to gain relationships with distributors to supply the Company's products. See Note 14—Acquisition, for discussion of the MPL acquired from CPS. The acquired intangible assets include technology, parts and service customer relationships, the MPL customer relationships, backlog and trade name. These intangible assets have estimated useful lives between one and ten years. The fair value assigned to identifiable intangible assets acquired has been determined primarily by using the income approach. Purchased identifiable intangible assets, except for backlog, are amortized on a straight-line basis over their respective useful lives and classified as a component of cost of goods sold or selling, general and administrative expenses based on the function of the underlying asset. Backlog is amortized on a per unit basis as the backlog units are sold and presented as a component of cost of goods sold.

        The manufacturing license provides the Company with the ability to manufacture recuperator cores previously purchased from Solar Turbines Incorporated ("Solar"). The Company is required to pay a per-unit royalty fee over a seventeen-year period for cores manufactured and sold by the Company using the technology. Royalties of approximately $72,800, $62,800, and $56,000 were earned by Solar for the years ended March 31, 2012, 2011 and 2010, respectively. Earned royalties of approximately $17,500 and $17,700 were unpaid as of March 31, 2012 and 2011, respectively, and are included in accrued expenses in the accompanying balance sheets.