An electricity generator having variable capacitors that each includes a repeatedly stretched and relaxed sheet (20, FIG. 3) of SM (stretchable material) and electrodes (22, 24) lying against opposite faces of the sheet, which includes a power extraction unit (110, FIG. 9) coupled to the electrodes of at least a pair of capacitors that are activated out of phase with each other. The unit directs an electrical charge (current) from a first pair of electrodes that have a higher voltage, to one or more second pairs of electrodes that have a lower voltage, to recharge the second pairs of electrodes. During the current flow electrical power is extracted by the drop in voltage of current passing through the unit. The power extraction unit can be provided with a control unit that has voltage detectors (54) and that selectively connect capacitors, based on their voltages, for maximum efficiency in generating power output.