The working principle of PV cells is based on the photoeffects. Cells are made up of one or more layers of p-n junctions of semiconductors, where they form electron-hole pairs, and the electromotive force is induced. Cells are divided into several types, of which the most common are those of crystalline silicon. In recent times there has been a development of thin film cells and organic cells. All work on the same principle of the photoeffect with the difference being the materials used for layers of p-n junctions. The main difference of rack-mounted PV systems and BIPV systems is in the way of installing the modules into the building. Other parts of the system are identical and are respectively: inverter, controller, batteries, wiring and protection equipment. In BIPV system modules are installed on rooftops, as windows, balcony fences, canopies and facades. Modules can partially or completely replace the conventional parts of the building. BIPV systems are often integrated parts of so called net zero buildings. The current state of the costs of BIPV and rack-mounted systems are generally equal, with the trend of prices decreasing in both systems.