A solar water-heating system usually consists of a solar collector on the roof to absorb sunlight, a hot-water cylinder and a back-up gas or electric water heater. In some systems, the water that is heated directly by the sun is the same water used by the occupants. In the system shown here, the household water is heated by an anti-freeze fluid, pumped continuously through tubes. When it reaches the collector it is heated by the sun. It then flows down to the hot-water cylinder where it heats the water supply. On days of limited sunlight, occupants may use a back-up heating system (usually gas or electric), which is also in the cylinder.