A fundamental characteristic of a photovoltaic system is that power is produced only while sunlight is available. For systems in which the photovoltaics is the sole generation source, storage is typically needed since an exact match between available sunlight and the load is limited to a few types of systems - for example powering a cooling fan.
In hybrid or grid connect systems, where batteries are not inherently required, they may be beneficially included for load matching or power conditioning.
By far the most common type of storage is chemical storage, in the form of a battery, although in some cases other forms of storage can be used. For example, for small, short term storage a flywheel or capacitor can be used for storage, or for specific, single-purpose photovoltaic systems, such as water pumping or refrigeration, storage can be in the form of water or ice.