In some of the sunny days we've had this summer, we've managed to try out hooking up Solar Panels as one of the power inputs to MoPi. The results are pretty good. A fairly decent panel setup can power even the Model B completely. In lower light levels or with less powerful panels, they can help increase battery life. We managed up to 19 hours with a Model B and 8 AAs by shutting down overnight:
That's pretty good. Those spikes are when the panels overpower the batteries completely. The next step is to work out how to charge in those conditions.