An ultra-low power (ULP), energy-harvesting system-on-chip, that can operate in various application scenarios, is needed for enabling the trillions of Internet-of-Things (IoT) devices. However, energy from the ambient sources is little and system power consumption is high. Circuits and system development require an optimal use of available energy. In this paper, we present circuits that can improve the energy utilization in an IoT device by providing improvements at critical points of the flow of harvested energy. A boost converter circuit, that can harvest energy from 10-mV input voltage and a few nanowatt of input power, makes more harvested energy available for the IoT device. A single-inductor-multiple-output buck-boost converter provides high-efficiency and low-voltage power management solution to put most of the harvested energy for system use. A real time clock and ULP bandgap reference circuit significantly reduce the standby power consumption. The proposed ULP circuits are developed in 130-nm CMOS technology. The combined effects of these circuits and the system design technique can improve the life-time of an example IoT device by over four times in higher power consumption mode and over 70 times in ULP mode.