Many independent power systems use more than one source of energy and thus are called hybrid systems. Most of the hybrid systems use PV in combination with a wind generator and optionally a diesel generator for the back-up-function. Hybrid systems ensure a continuous and reliable electricity supply. They also provide enough power to satisfy modern domestic needs (lighting, communication, refrigeration, water supply) as well as public services (health centres, schools) and the development of a local economy (small industries and related services such as telecommunication towers and water irrigation systems). Hybrid systems can also be used for centralized village power supply by serving a mini-grid.