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.
In any photovoltaic system that includes batteries, the batteries become a central component of the overall system which significantly affect the cost, maintenance requirements, reliability, and design of the photovoltaic system. Because of large impact of batteries in a stand-alone photovoltaic system, understanding the properties of batteries is critical in understanding the operation of photovoltaic systems. The important battery parameters that affect the photovoltaic system operation and performance are the battery maintenance requirements, lifetime of the battery, available power and efficiency. An ideal battery would be able to be charged and discharged indefinitely under arbitrary charging/discharging regimes, would have high efficiency, high energy density, low-self discharge and be low cost. These are controlled not only by the initial choice of the battery but also by how it is used in the system, particularly how it is charged and discharged and its temperature. However, in practice, no battery can achieve the above set of requirements, even if normally dominant requirement for low-cost is not considered. This chapter provides an overview of battery operation and use for photovoltaic systems.