A generator unit to harvest energy from motion is disclosed which comprises a support (6) with a connection member (7) for attachment to a movable object (8), at least a first body (3) and a second body (9) supported by the support and configured to move relatively to each other and with a varying, preferably magnetic field (4) spatially in between, one of the first or second body comprising at least one electricity generating member (2) of piezoelectric, electromagnetic or electrostatic type to produce electricity when mechanically deformed or moved, and a field engagement part (5) configured to move in the varying field when the first and second bodies move relatively to each other and generate a varying mechanical deformation or movement of the electricity generating member.