Province of North Sulawesi in Indonesia is estimated to have geothermal energy as much as 865 MWe. One of the geothermal fields in North Sulawesi is located in Lahendong, which contains potential energy about 530 MW (Pertamina Geothermal Energy 2009). Currently, Lahendong geothermal field is being developed by Pertamina Geothermal Energy (PGE), a subsidiary of PT Pertamina, the state-owned oil and gas company of Indonesia. Two geothermal power stations have been operated with installed capacity of 20 MW each (Unit 1 and Unit 2) in that geothermal field. There is also a 20 MW power station (Unit 3) that has been undergoing operational testing since April 2009. This book assesses the gross electrical power that can be generated from the production wells in the Lahendong geothermal field.