The light bulb feature on most solar charge controllers are usually called "load" or something similar. Typically it is designed for lighting applications. When the controller senses a charging current from your panel, it will turn the load off. Then, when it stops sensing power from your panel, it realizes that it must be dark outside, so it turns the load on. Usually you can also set the load to be on for 24 hours if you want. Other more sophisticated charge controllers can let you define which hours you want the load on. If the load is never used (which is very common), then the charge controller will still regulate charging with no excess power produced because it will disconnect the panel when the battery is charged.
As always, please let us know if you have any questions at Sales@CuttingEdgePower.com