In non-steady-state experiments, the electrical response of devicesbased on disordered organic semiconductors often shows a large transient contribution due to relaxation of the out-of-equilibrium charge-carrier distribution. We have developed a model describing this process, based only on the parameters describing the d.c. mobility andon Monte Carlo calculations of the effective conduction energy level. The model successfully predicts the relaxation-enhancement of thedifferential capacitance of sandwich-type devices based on a polyfluorene-copolymer. Generalization of the approach is expected to enable efficient modeling of relaxation effects in other types of experiments.