Powerable working machine, in particular mowing working machine, for mounting on a carrier vehicle, having - a working head comprising a powerable working element, - a fastening device comprising an extension arm, to the front, free end of which the working head is attached, - a head motor, in particular with a continuous power of more than 10 kW, which is arranged at the working head - an energy source for providing a form of energy which can be used by the head motor, - energy lines connected to the head motor on the one hand and to the energy source on the other hand for supplying energy to the head motor, - a control unit which controls at least all the movable elements of the working machine, wherein - the head motor is an electric motor - the energy lines are electric lines, - the energy source is a current source, in particular a generator and/or a battery and/or an electrical connection to the internal supply-system of the carrier vehicle or a combination of these elements, characterised in that a) the fastening device comprises - a fastening base which can be attached to the carrier vehicle with a transverse fastening rail running in the transverse direction, along which the extension arm can be moved, - by means of a slide, on the upper side of which the extension arm is fixed with its vehicle-side end, - and is rotatable about an upright axis, - wherein the extension arm consists of a plurality of arm parts (9a - d), that are jointly connected to each other, - wherein the plurality of joints (15a to 15e) are pivot axes extending parallel to each other and transverse to the direction of extension of the rod-shaped arm parts (9a to 9d), b) the battery electrically connected to the head motor is - either a backup battery electrically connected to a generator and having a capacity of not more than 20 kWh, which is mounted on the fastening base for the boom arm - or a supply battery of a capacity of at least 20 kWh, which is - located apart from the fastening device and the working head carried thereby, and c) the head motor has liquid cooling. - a working head comprising a powerable working element, - a fastening device comprising an extension arm, to the front, free end of which the working head is attached, - a head motor, in particular with a continuous power of more than 10 kW, which is arranged at the working head - an energy source for providing a form of energy which can be used by the head motor, - energy lines connected to the head motor on the one hand and to the energy source on the other hand for supplying energy to the head motor, - a control unit which controls at least all the movable elements of the working machine, - the head motor is an electric motor - the energy lines are electric lines, - the energy source is a current source, in particular a generator and/or a battery and/or an electrical connection to the internal supply-system of the carrier vehicle or a combination of these elements, a) the fastening device comprises - a fastening base which can be attached to the carrier vehicle with a transverse fastening rail running in the transverse direction, along which the extension arm can be moved, - by means of a slide, on the upper side of which the extension arm is fixed with its vehicle-side end, - and is rotatable about an upright axis, - wherein the extension arm consists of a plurality of arm parts (9a - d), that are jointly connected to each other, - wherein the plurality of joints (15a to 15e) are pivot axes extending parallel to each other and transverse to the direction of extension of the rod-shaped arm parts (9a to 9d), - a fastening base which can be attached to the carrier vehicle with a transverse fastening rail running in the transverse direction, along which the extension arm can be moved, - by means of a slide, on the upper side of which the extension arm is fixed with its vehicle-side end, - and is rotatable about an upright axis, - wherein the extension arm consists of a plurality of arm parts (9a - d), that are jointly connected to each other, - wherein the plurality of joints (15a to 15e) are pivot axes extending parallel to each other and transverse to the direction of extension of the rod-shaped arm parts (9a to 9d), b) the battery electrically connected to the head motor is - either a backup battery electrically connected to a generator and having a capacity of not more than 20 kWh, which is mounted on the fastening base for the boom arm - or a supply battery of a capacity of at least 20 kWh, which is - located apart from the fastening device and the working head carried thereby, and - either a backup battery electrically connected to a generator and having a capacity of not more than 20 kWh, which is mounted on the fastening base for the boom arm - or a supply battery of a capacity of at least 20 kWh, which is - located apart from the fastening device and the working head carried thereby, and c) the head motor has liquid cooling.