NOVELTY - The device (8) has a temporary fastening unit (15) for fastening a support (9) having photovoltaic cells (11) on a trailer (3) of a tractor. An electrical connecting unit (18) is provided for electrically connecting the photovoltaic cells to the tractor. The support is designed to be in a working configuration in which the support is spread onto a trailer roof (4) and a storing configuration in which the support is stored on the tractor in a compact manner. The support comprises flaps (12-14) that vertically close vertical surfaces (5, 6) of the trailer in the working configuration.    USE - Solar energy recovery device for a truck (claimed).    ADVANTAGE - The device can effectively increase the electricity supplied to the truck. The support comprises the flap placed vertically close to a vertical surface of the trailer in the working configuration, so that the support can be positioned and maintained in a proper location. The main portion extends over the whole width of the trailer, so that the device is easy to handle and store by a driver. The device can be installed on the trailer easily and temporarily, and can be transferred from one trailer to another trailer easily, so that the solar panel capacity can be increased in order to fulfill the needs of electricity powered elements in the truck and the overall fuel consumption can be reduced and the driver comfort is improved, as need the for starting the engine up is avoided and engine noise is eliminated. The device can be stored in the compact configuration in the tractor.    DETAILED DESCRIPTION - The support is made of canvas or tarpaulin.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a truck trailer equipped with a solar energy recovery device having a support in a working configuration.    Trailer (3)    Trailer roof (4)    Vertical surfaces of trailer (5, 6)    Solar energy recovery device (8)    Support (9)    Photovoltaic cells (11)    Flaps (12-14)    Temporary fastening unit (15)    Electrical connecting unit (18)