An integrated collector storage (ICS) solar water heater was designed and studied with an emphasis on its geometric characteristics optical properties. The ICS system consists of a single cylindrical storage tank placed in a CPC reflector composed of three parabolic branches. The suggested design aims to cover the need of hot water of a family composed of four persons during the few sunny days as well as during the night. A theoretical optical model based on the average number of reflections was used. The comparison of the thermal efficiency performance between this system and two other systems of solar water heaters, each of them was composed of a storage tank placed in symmetrical and asymmetrical CPC reflectors, was done. The results show important thermal efficiency despite the simplicity and the little cost of our collector.