A typical Solar Domestic Hot Water (SDHW) system consisting of solar collectors and a water storage tank operating in fully mixed regime is considered. Optimal control operation is assumed. The objective is to maximize the net daily heat provided by the solar collectors. Meteorological data measured during year 2009 in Timisoara (Romania, Southeastern Europe) are used. Several days with more or less stable radiative regime are selected. Most results correspond to summer and spring days with daily relative sunshine larger than 0.4. The pumping energy increases by decreasing the daily relative sunshine. During more stable days the pump in the primary circuit operates a longer period of time than during less stable days. The opportunity to use the SDHW system for replacing a classical energy source depends on the stability of the radiative regime. The conclusion is that the dependence of SDHW systems' performance on the stability of the radiative regime is a complicate function of the specific performance indicator and the available amount of solar energy. (C) 2015 Elsevier Ltd. All rights reserved.