Abstract: A mathematical model is presented to study the performance of thermal storage for excess solar energy at day to heat a living room using a tank of water as thermal storage. The study studies the relationship between the room temperature and the appropriate tank geometries through the day hours. Two storage geometries have been studied; the tank surface area which was varied from 6-12 m2 and the storage water mass per unit area of the collectors (50-70 kg m-2) on the storage temperature. The study is based on several years collected data including solar intensity, ambient temperature and the wind speed which were measured in Tikrit city-North West of Iraq-in January. The solar collector was constructed from multiple standard units; each unit is (1 m) width and (2 m) length. Each unit consists of six copper pipes with 10 mm diameter and 2 m long covered by galvanized steel sheet 1 mm thickness and a glass cover of 3 mm thickness. The mathematical model was converted to a simulation computer program accounting for all effective parameters on heat transfer in the heated room space, the collector and the storage tank. The results show that thermal storage has an important effect on raising the room temperature for many hours in the evening and nearly reaching the comfortable conditions requested at night.