An apparatus comprises an output port for a circuit load, a first input port for an energy harvest source, an input/output port a second energy source, a first circuit path from the energy harvest source to the second energy source at the input/output port and to the variable load at the output port, a second circuit path from the second energy source to the output port, a cold start circuit that produces a first voltage level at the output port by charging a capacitor at the output port using energy of the energy harvest source, and a main converter circuit that produces a second regulated voltage level at the input/output port using energy of the energy harvest source when the voltage at the output port capacitor is above a specified voltage value and uses the energy of the capacitor at the output port during startup of the main converter circuit.