This paper presents a fully integrated energy harvester that maintains > 35% end-to-end efficiency when harvesting from a 0.84 mm(2) solar cell in low light condition of 260 lux, converting 7 nW input power from 250 mV to 4 V. Newly proposed self-oscillating switched-capacitor (SC) DC-DC voltage doublers are cascaded to form a complete harvester, with configurable overall conversion ratio from 9x to 23x. In each voltage doubler, the oscillator is completely internalized within the SC network, eliminating clock generation and level shifting power overheads. A single doubler has > 70% measured efficiency across 1 nA to 0.35 mA output current ( > 10(5) range) with low idle power consumption of 170 pW. In the harvester, each doubler has independent frequency modulation to maintain its optimum conversion efficiency, enabling optimization of harvester overall conversion efficiency. A leakage-based delay element provides energy-efficient frequency control over a wide range, enabling low idle power consumption and a wide load range with optimum conversion efficiency. The harvester delivers 5 nW-5 mu W output power with > 40% efficiency and has an idle power consumption < 3 nW, in test chip fabricated in 0.18 mu m CMOS technology.