One of the great challenges to using thermoelectric generators (TEG) for power generation is large-scale utilization. It is difficult to manufacture TEG systems even at the scale of a few kilowatts (KW). There have been many reports on laboratory experiments measuring TEG power output at different flow rates of water, different temperatures, and other different conditions, but there have been few field tests utilizing geothermal wells. To this end, we designed a geothermal-TEG apparatus that has a six-layer modularized design to allow expanded power production. After demonstrating the TEG’s performance in the laboratory, we tested the apparatus in the field at a well located in the Bottle Rock geothermal field in The Geysers, CA, USA. The whole six-layer TEG device could generate about 500 W electricity with a temperature difference of about 152 °C between the hot and cold fluid manifolds, while each TEG chip could generate about 3.9 W. The steam pressure at the inlet of the TEG apparatus was about 122 psi, close to the wellhead pressure of 125 psi. After optimizing the field infrastructure, the six-layer TEG device could generate electricity without any leak at the wellhead pressure of 125 psi and the temperature over 176 °C (349 °F). The field test of the six-layer TEG device at Bottle Rock geothermal power plant was considered successful, and plans have begun to design and build a TEG device that could produce power up to 20 KW.