In a solar water heating system, a heat-transfer fluid flows through the tubes of solar collectors, where it is heated and transfers the heat to the household water. The heated water is then stored for use in a tank. A solar water heating system can be used as the sole source for hot water or may include a back-up conventional system to meet heavy or unusual hot water requirements throughout the year. Systems are usually sized according to the number of bedrooms, people and household water needs. There are several different configurations of solar water heating systems. In general, however, there are two main types: active systems which have pumps and controls to deliver solar heat to the storage tank, and passive systems which combine the collector and storage in a single unit.