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.