SolarPower is a just-for-fun app, you can prank your friends. Simply start the app, put your phone in a well lit place and watch your battery charge using the solar panel. The app uses the brightness sensors of the device, the stronger of the light, the faster it charges!
NOTE: It is technically not possible to generate electricity and charge your battery. Never put it for a long time to the direct sunlight, because the high temperatures can damage your phone!