The Ipad Mini is very energy efficient and therefor perfect to be charged by a solar panel. Here's a practical real-life example how much solar panel surface is needed to charge the Ipad Mini Battery.
Ipad Mini on solar power
A simple Solar panel with a car charger can charge the Ipad Mini battery - given there's a decent amount of sun available;
This panel delivers 200 mA at 12.7 Volts (2.54 Watt) and is made to charge a Thuraya Satellite phone. To the right a 12V USB car charger delivering 5 Volts to the Ipad Mini. (The USB Car charger is mentioned further below.)
The solar panel delivers enough current to charge the Ipad Mini battery without the "Not Charging" notification.
With the Ipad Mini battery fully charged this Solar Panel is enough to power the Ipad Mini. When connected the iPad Mini shows the AC Mains plug in the upper-right corner of the display.
Simultaneous battery charging (full cycle) and using the Ipad Mini the Solar Panel Surface should at least be 3x as large than the panel shown above. This is roughly 3 times the surface of the Ipad Mini itself.
A few examples of portable solar panels;
This solar panel connects to a standard 12 volt car jack. The USB car charger connected to the solar panel is nothing special and delivers 5 Volts DC at 1 Amp maximum.
Do note that full sunshine is required to get the power needed to get the Ipad Mini battery charging going. Using a small solar panel like this on cloudy days won't do anything.
Read about the Ipad Mini power consumption and the Ipad Mini USB car charger.