In the United States, Citizen is best known for its Eco-Drive range of watches. Eco-Drive watches use a battery recharged by a solar panel hidden under the watch face. In the rare and discontinued Eco-Drive Duo series, the solar power was supplemented by an automatic quartz power source. There was also an Eco-Drive Thermo model that exploited temperature differentials between the wearer's skin temperature and ambient temperature to recharge the battery. However, the only Eco-Drive system currently described on the Citizen Watch Official Web Site is the one depending solely on light to recharge. The Eco-Drive feature is similar to the Tough Solar battery charging system used in G-Shock watches by Casio. All Citizen Eco-Drive models are made in Japan but may also be assembled in China.