Results of a study on the evaluation of potential for geothermal energy development and utilization in California are reported. Volume I of the two-volume report deals with hydrothermal resource costs and economics in terms of various critical parameters pertaining to specific resource locations in California. The important economic factors that influence the development and growth of hydrothermal resources and the probable costs of a hot-water demonstration power plant are also considered. A set of near-term actions and their estimated impacts on hydrothermal resource economics are also discussed. (JGB)