There are Youtube videos about energy harvesting. From what I can see an experimenter is glad if he gets a few uA @ 4 V.
Remember the simple 'crystal' radio set? With no battery you can hear broadcasts in headphones (high impedance type). If you're near a transmitter then you'll generate more power.
The load current is mainly limited by the available input voltage and step-up transformer ratio. Review the respective data sheet diagrams. There are diagrams for Vout = 4.5V, not so far from the intended operation point.