Plasmas in electron cyclotron resonance ion sources (ECRIS) are collisionless and can therefore be simulated by only following the motion of electrons in the confining static magnetic and oscillating microwave field of ECRIS. The experimental performance of three different ECRIS is successfully compared to calculated spatial electron (plasma) and electron energy density as well as to the energy spectrum and the average energy of the electrons. Further simulations suggest a new and better design of an ECRIS, the good experimental performance of which corresponds to the predictions.