Storage is used in PV systems to increase the amount of time that the PV system can be used to power a load. Batteries are the most common type of storage in a PV systems. However, in specific types of systems or applications, other storage components can also be used. For example, in water pumping systems, the amount of battery storage can be greatly reduced or eliminated if extra water is pumped and stored in a water tank for use in cloudy periods.

In stand alone electricity generating systems, some form of storage is needed unless the load is exactly matched to the time during which the sun is shining. (Such an exact match is rare and limited to a few types of systems - for example powering a fan for cooling or in some cases water pumping for irrigation). In stand alone systems, storage is needed not only to power loads at night, but also allow a load to operate during cloudy weather. The number of days of storage needed depends on the weather pattern at a particular location, with cloudier locations needing more storage. In systems with a large amount of storage, and additional utility of the storage system is that is can buffer the system against periods of low insoltion, such as in winter. For example, in telecommunications systems that require high reliability, a large battery bank can allow high reliability without requiring the PV array to be sized to meet the worst possible insolation conditions. In general, the larger the amount of storage included, the less sensitive the system will be to periods of low insolation, and the more relibable the power availablity will be. The figure below shows how the power availability increase with increasing storage.

Battery state of charge over a year showing the battery discharge overnight, during cloudy weather and seasonal variations.

In systems connected to the utility grid electricity supply, storage is typically not needed. PV power is used when the sun is shining, and at night or during periods of cloudy weather, the grid provides the electricity. However, even in grid-connected systems, storage can be included, not to increase the reliablity of having power as in a stand-alone system but rather to increase the value of the PV-generated electrcitiy. In the load seen by many utlity companies, an air conditioning load occuring on summer afternoons increases the overall load that the utility must supply. These peaks in the load are significantly more expensive to supply power for. Since the power poutput from PV is typically largest during summer months, the output from the PV system can well-matched to the peak load of the generated electrciity is stored for a few hours. The use of storage for this application is called peak shifting and is shown in the figure below.