For the design of electronic lens systems following a 180 degrees monochromator, the size and distribution of the space charge which tends to widen the beam must be known. The charge contained within a narrow bundle cut out of the input beam depends on the energy of the main path, the input angle and energy spread of the total input current, and on the distance from the entrance slit. A first-order model is established which allows an analytical calculation of the space-charge distribution as a function of the distance from the entrance slit for electron beams of uniform distribution over energy and input angles.