Energy demand does not always coincide with energy production, so many PV systems include electrical storage batteries. Batteries store electrical energy when it is produced by the array and supply energy to electrical loads when needed. In other words, you charge your batteries with the sun during the day and use the energy from your batteries at night.
This is an example of an "off the grid" system, with the buildings electrical system operating without being tied to the utility grid. These systems are most ideal for remote locations such as cabins or vacation homes where connecting to the utility grid is a costly challenge.