We present a description of Monte Carlo simulations of the pulse height energy response of CCD x-ray detectors. Effects of event charge splitting between pixels are accounted for, and the variation of energy resolution and detection efficiency with event selection criteria are discussed. We show that there are important implications for background rejection efficiency, on-board calibration, and ground-based data reduction. In orbit energy resolution may become degraded by traps created by radiation damage. We present an analysis of trapping and emission time scales, which allows us to predict the energy resolution for a wide range of device operating conditions.