Compact apparatus for the horizontal assembly of photovoltaic panels with cells, intended to progressively assemble on a tray the semi-finished components which make up each panel such as: - supporting backsheet, - interconnection elements of the conductive ribbons type, - photovoltaic cells, - layers of encapsulating material, - front glass, also different every time to individually form panels with cells having a double or single contacting face, in such a way that each panel is completely assembled and ready for the subsequent rolling phase which can be carried out in conventional furnaces, said compact apparatus being characterised in that it consists of one single closed-chamber multi-function workstation integrating working and conveying means (108-110, 120-2, 130-4) intended to carry out concomitantly and in a coordinated way, on one working surface only, the operations of assembly of the panel; said compact apparatus, being intended to assemble said panel by progressive superimpositions on the tray which is introduced in correspondence of said working surface; said compact apparatus being of the semi-automatic type, since all the assembly operations inside said closed chamber are automatic and coordinated with each other, and on the other hand the accessory operations of loading-unloading carried out outside by the operator are manual; and wherein horizontal conveying means, in the form of conveyor belts, protrude frontally from said closed chamber in correspondence of side openings placed at the height of the working surface, one opening being shaped as a horizontal slit for the crossing of trays on a first conveyor belt, and the other opening for the crossing of the cells being on a second conveyor belt; and wherein within said closed chamber, on the sides of said working surface, there are: a first loading zone for said cells, a second loading zone for the conductive ribbons which is opposite to the first zone, a pair of longitudinal rails adjacent to said working surface and to said loading zones (111, 113) with one rail opposite to the other; said pair of longitudinal rails for longitudinally guiding the coordinated and independent movements of at least two mobile gantries (120, 130, 151-4) which slide horizontally above said working surface from one loading zone to the other, being of the roto-translation Cartesian robot type, simultaneously and independently intended to translate and also to rotate around the vertical axis; and wherein each of said mobile gantries (1 20, 130) is of the telescopic type and is individually constrained to said longitudinal rails with a pair of hinge-sliding blocks independent and coordinated with each other in such a way as to control the horizontal sliding as well as the rotation around the vertical axis, thus facilitating the alignments on the working surface; said mobile gantries (120, 130) being individually equipped for the purpose of the assembly of said panel with working means (121-2, 131-4) which are mobile, to translate longitudinally along the gantry operating at least on the vertical axis; said first mobile gantry, at least comprising: pneumatic means for the taking-laying of the cells and of the front glass combined with means for the detection and automatic control of the positions; said second mobile gantry at least comprising: means for the automatic dispensing of a conductive adhesive combined with means for the detection and automatic control of the positions also with pneumatic means for the taking-laying of the conductive ribbons and means for prefixing by localised heating; said working and conveying means (108-110, 120-2, 130-4, 151-8) being managed by a logic control unit in such a way as to carry out coordinated operations according to the assembly cycle required for the panel; said coordinated operations being concomitant, that is to say, carried out by said means (108-110, 120-2, 130-4, 140) on the same working surface and also being simultaneous if performed in the same moment. - supporting backsheet, - interconnection elements of the conductive ribbons type, - photovoltaic cells, - layers of encapsulating material, - front glass,