A water sterilizer powered by solar energy is described. A first check valve allows an increment of untreated water to flow from a reservoir into a boiling chamber where it is heated to boiling, and thus sterilized, by focused solar energy. A second check valve with a higher cracking pressure than the first valve remains closed, holding the increment of untreated water in the boiling chamber until the pressure of steam forces the second valve open, thus evacuating the chamber of now-sterilized water. Once the chamber is evacuated, the second valve automatically closes, and the first valve automatically opens, allowing another incremental sterilizing cycle to begin.