An off grid solar system is a solar system that operates without any utility backup. These systems are typically remote where the cost of running a utility connection is more expensive then purchasing an off grid solar system. I have design and installed off gird systems that power remote data loggers, flow meters for gas lines, radiation monitoring systems, remote cabins, sign lighting, area lighting and much more. In this example any DC load can be powered with solar.
In this example the solar module is wired through a fuse to a charge controller. The charge controller prevents the solar module from over charging the battery. Most modern charge controller will also have a built in load controller. The load controller will disconnect the load of the battery voltage drops too low. This will prevent the load and the battery from being damaged. If a battery is discharged too low the acid inside becomes more of a water solution and can easily freeze. With some charge controllers the load controller can be used as a dusk to dawn timer where the solar module is used to determine if the sun is up or down and the load (light) is turned on or off. The energy from the solar module is stored in the battery and is consumed by the load. The correct balance of power in and power out needs to be maintained for system reliability.