We briefly sketch our lattice definitions of a canonical and grand canonical ensemble of QED with respect to charge, and then present results of MC-simulations we did in both ensembles in order to study the chiral phase transition of non-compact QED with light staggered fermions. The fermion chemical potential is found to be a sensitive order parameter of the transition. In the chirally broken phase, the charge dependence of the chemical potential appears quite different from that of massive free fermions. ((orig.)).