February 24, 2010
Board of Directors
717 Texas Avenue
Houston, Texas 77002
We are providing this letter to you for inclusion as an exhibit to your Form 10-K filing pursuant to Item 601 of Regulation S-K.
We have audited the consolidated financial statements included in the Companys Annual Report on Form 10-K for the year ended
December 31, 2009 and issued our report thereon dated February 24, 2010. Note 3 to the financial statements describes a change in accounting principle from the composite depreciation method to the component depreciation method for rotable
parts at the Companys natural gas-fired power plants. In addition, also described in Note 3 to the financial statements is a change in accounting principle from the units of production depreciation method to the straight-line depreciation
method for the Companys geothermal assets. It should be understood that the preferability of one acceptable method of accounting over another for the depreciation of rotable parts and the depreciation of geothermal assets has not been
addressed in any authoritative accounting literature, and in expressing our concurrence below we have relied on managements determination that these changes in accounting principle are preferable. Based on our reading of managements
stated reasons and justification for these changes in accounting principle in the Form 10-K, and our discussions with management as to their judgment about the relevant business planning factors relating to the changes, we concur with management
that such changes represent, in the Companys circumstances, the adoption of a preferable accounting principle in conformity with Accounting Standards Codification 250, Accounting Changes and Error Corrections.
Very truly yours,
/s/ PricewaterhouseCoopers LLP