We introduce a compact, lightweight and highly efficient multi-resonator rectenna (rectifying antenna) designed to harvest the RF energy really obtainable in humanized environments. Such sources, radiated from cell phones, radio transmitters and Wi-Fi equipments, are ubiquitously available but have very low power densities, at different frequency bands, with unknown directions of incidence and polarization. In order to harvest a significant quantity of energy it is mandatory to put a very special care in the design of each part of the receiving/storing system. For this purpose a combination of resonant antennas, each one designed for a specific application-dependent frequency band, is optimised together with the rectifying circuit and the load. This is accomplished by a rigorous design tool, based on the concurrent use of nonlinear and electromagnetic CAD methods. Multi-source non linear simulation of the harvester in realistic operating conditions predicts a DC power of a few hundred W, which represents the typical energy requirement of a sensor node