How-To: Solar-Powered Raspberry Pi FTP Server
This build uses a custom 3D-printed / hand assembled case from RPi hacker Cottonpickers, which comes attached to the back of a 300mA solar panel, and has a bunch of nice features already built-in. Once you’ve got your Pi running on sunlight, David Hayward over at CNET UK will show you how to configure the Pi so you can access an external hard drive attached to one of its USB ports via FTP.