The Solar Panel is an Industrial Craft 2 generator. Solar Panels will generate EU so long as they have a direct line of sight to a clear, day-lit sky. The Solar Panel will not generate EU if they are placed under a solid block or if it is nighttime, and will generate far less EU during rain and thunderstorms.
Solar Panels generate 1 EU/t in optimal conditions. Solar Panels are also used in the crafting of the more efficient Low Voltage Solar Array, which is in turn used to craft the Medium Voltage Solar Array, and finally the High Voltage Solar Array. The Solar Panel is also part of the recipe for the Solar Helmet, which recharges items worn in the Torso slot of the inventory, such as an Electric Jetpack or a BatPack.
The GUI of the Solar Panel features only a single slot. Tools and batteries can be placed in it to recharge them.