Since solar panels only produce electricity when sunlight is present, a battery is needed to store the energy for use on a continuing basis. In many systems, the solar panel is used as a "trickle charger" for the battery, which is the primary source of power for the application. The size and output of the battery is an important part of providing reliable power to an application. The battery, like the solar panel delivers DC electrical current.
The size, configuration, and type of battery are all important in matching it to the expected environment for the solar application. Temperature variations and power demand cycle for the application can impact the choice of batteries used. Lead acid, nickel hydride, and lithium ion are all battery types that have distinctive performance, cost and useful life characteristics that need to be considered in each solar application.