I took a 10-day photo/rafting trip through the Grand Canyon a few years ago and only used one battery after taking several thousand images with my D3x. While that camera's battery is substantially bigger than others, it might be better to just carry extra batteries and save the expense and logistics (weight, volume, clouds, etc.) of solar panels, transformers, wires, etc. If you're being resupplied, the crew could bring freshly charged batteries and they could take out the exhausted ones.