Harvesting energy from non-stationary, multi-frequency mechanical vibrations using a tunable electrical circuit. In an embodiment, an apparatus for converting vibrational energy to electrical energy includes a vibrational energy harvester having a transducer for generating time-varying electrical signals in response to environmental vibration; at least one power storage device; a switching network operably coupled between the transducer of the vibrational energy harvester and the at least one power storage device, wherein the switching network includes a plurality of switching elements each defining a switchable current path that is controlled by a control signal supplied to the respective switching element; and electronics configured to generate the control signals for supply to the switching elements of the switching network, the electronics including first circuitry, second circuitry, third circuitry, and fourth circuitry.