The button on the bottom when pushed down shows how much of a charge the solar charger has. The red lights on the bottom turn to green after is is charged. The AC adapter plugs into the wall and charges the unit and also provides a power source when sunlight is not available. Sun charges the lithium ion batteries inside the solar charger which feeds into the camera. Because the charger does not charge the batteries inside the camera you can use alkaline batteries in the camera. When the solar charger is plugged into the camera the camera will only use its internal batteries when there is no power coming from the solar charger.