1. A water heater comprising: a water storage tank; a heat source for heating the water stored in said water storage tank; a cold water input conduit; a controllable cold water input valve operable to open and close said cold water input conduit for allowing or preventing the flow of water into said water storage tank; the cold water input conduit allowing unheated water to enter said water storage tank through said cold water input valve; a warm water output conduit of variable length operable to allow heated water to exit said water storage tank; an actuator operable to vary the length of said warm water output conduit to accommodate for different volumes of water storage within said water storage tank; and a sensor operable to quantify the amount of water stored in said water storage tank and to provide a signal indicative thereof for controlling said cold water input valve.