The sensor is often supplied with electrical power. For sensors studied in previous chapters, the electrical power comes from batteries, which have limited lifetime. Therefore, several technologies for recharging batteries have been developed recently. Such technologies utilize energy sources in environment, such as sunlight, electromagnetic radiation, thermal, and vibration. A sensor with such a technology is called the energy-harvesting sensor, that is, an energy-harvesting sensor is able to convert energy of other types, in environment, to electrical energy. Among other energy sources, the sunlight is the most available one outdoor. Solar technologies are developed quite fast and now already reach conversion efficiency of 15%.