This work presents a low-power battery-less energy harvesting system for Internet of Things (IoT) applications. A dual-mode DC-DC converter is used to harvest the energy of a micro-scale photovoltaic (PV) transducer and provides energy, in the boost mode, to a super capacitor for storage. In the buck mode, the DC-DC converter provides energy to the load on demand. A piezoelectric transducer is used as a secondary input source to guarantee system’s self-start-up. The proposed system includes a smart control to adaptively adjust the number of sensors at the load, leading to better usage of the available energy at any given time. The implementation also includes a programmable switch sizing to optimize the overall system’s efficiency for different load conditions. A maximum power point tracking is used to extract the maximum power of the PV transducer under various illumination conditions. The system has been implemented in a CMOS 130nm technology and tested in various modes of operation. Self-start-up has been verified and a peak efficiency of 90.5% has been measured.