Sizing and applicability of solar combisystems providing both space heating and domestic hot water are investigated in this paper. The analysis is based on an analytical daily model to predict the average yearly system performance. The study indicates that increasing the collector area in a solar combisystem for higher solar fractions could be economically justified in average or older building stock in northern and central Europe but not yet in low energy or very energy efficient buildings nor in a more southern climate. Increasing the size of the heat storage much beyond the daily capacity proved not to be well justified in solar combisystems. (C) 2004 Elsevier Ltd. All rights reserved.