Stand-alone systems, which are also called off-grid systems, rely on solar power only. These systems can consist of the PV module and a load only or they can include batteries for energy storage. When using batteries charge controllers from the PV modules when they are fully charged, and may disconnect the load to prevent the batteries from being discharged below a certain limit. The batteries must have enough capacity to store the energy produced during day to be used at night and during periods of poor weather