A device able to generate electrical power through relative rotational motion of first (370, 380) and second (200, 230) principal components around an axis of rotation (220); wherein: the first and second principal components comprise an arrangement of piezoelectric elements (380) and permanent magnets (230, 370) such that the interaction between these magnets and piezoelectric elements, in use, makes it possible to generate electricity; and wherein: the second principal component (200, 230) comprises a centre of mass offset from the axis of relative rotation (220) such that the response of the second principal component (200, 230) to either gravitational or inertial forces is a relative rotation of the second principal component (200, 230) in relation to the first principal component (370, 380); the first principal component (370, 380) being fixedly attached to the moving host structure (100).