In this paper we present a 4-mm2 battery-less temperature sensor, designed to collect and periodically transmit information to an ambient monitoring system. It consists of a 0.35-µm BCD SOI technology based chip including an ultra-low power integrated circuit and an energy harvesting supply unit made by 34 series-parallel connected photovoltaic cells. Each cell consists of a trench insulated p-n junction. Series connections among cells are possible because of the lack, guaranteed by the SOI insulation, of the substrate parasitic diode, typical in standard CMOS technology. The system is fully compliant with real-time applications and can work in low illumination conditions and with large variations of the photovoltaic voltage.