Above the absorbent surface is a transparent cover, usually of glass or plastic, that allows the sun's rays to pass through but prevents heat from escaping. The tubes through which the fluid circulates are in contact with the absorbent surface resulting in heat transfer to the fluid.

The simplest and most widespread solar power system is the solar water heater. Its principle of operation is simple and is based on the fact that the water heated in the collector expands and becomes lighter than the lower water temperature of the tank. This difference in water density results in the natural circulation of the collector medium and the transfer of hot water to the storage tank. At the same time the cold water of the tank is pushed towards the collector. A prerequisite for the natural circulation of water is to place the tank at a point higher than the collectors. If lower, the water circulates with the help of proper automation.

Another application that is spreading in the European market is the combination of hot water production and space heating with active solar systems. The use of these systems is considered both technically and cost-effective if combined with proper design / construction of the building (good insulation, exploitation of passive solar benefits, etc.) and user cooperation. Active solar systems can also be used for space cooling, with the appropriate technologies (absorption cooling machines). Apart from home use, which is the most widespread today, active solar systems can be used wherever low temperature heat is needed (such as space heating / cooling, water desalination systems, pool water heating, etc.).

The technology of central solar systems is considered as a very cost-effective solution for the production of hot water and is widely used in homes, hotels, restaurants, industries, etc.