The behavior of the solar still-storage tank system, designed and installed in Solar & other Energy Laboratory of NCSR "Demokritos", is investigated through its operation under real conditions by keeping tank water temperature constant in different levels. The solar still is of greenhouse asymmetric type, the basin of which is in direct contact with a hot water storage tank as an integral part of the still, ensuring thus continuos transfer of heat from the hot water tank to the basin saline water. The thermal behavior of the system was investigated by several series of measurements with the saline water temperature kept constant in different temperature levels. It has been found that this design leads to higher water outputs as the water temperature increases but more important is the shift of water production towards night periods. Moreover, water production rate is almost constant through out the whole period. It has also been found that coupling solar still with hot water tank changes the whole behavior of the solar still in such way that in the coupled system solar still acts as a condensing unit and not also as a energy collecting unit, the case of the solar still itself. For this, changes in solar radiation do not affect the system productivity. Finally, experimental data concerning day and night distilled water production are compared with the ones already taken in the solar still only, indicating the effectiveness and benefits of coupling the storage tank with the still. It is proven that the operation of the solar still-storage tank system is advantageous over the still itself regarding production quantity, distribution, rate pattern with the ability to exploit in an optimum way available heat sources nearby such as waste heat, geothermal energy, conventional heat, etc.