A detailed energy analysis of an ice-dust-sample under interplanetary space conditions is presented. It is based on measurements obtained during a comet simulation (KOSI) experiment. Results of the analysis show that visible and thermal reradiation from the sample surface, the sublimation energy of the ices and the increase of internal heat are the most important contributions to the energy balance. At the beginning of irradiation a relatively high ice sublimation rate is observed. Dust particles remaining on the surface, build up a nonvolatile mantle, producing an obstacle for heat flow inwards and gas flow outwards. Consequences for cometary nuclei models are discussed.