Another power option is the Velogical dynamo. I recently purchased one, have put a few miles on it, and am impressed. Excellent customer service. I plan to use it for charging a backup battery for a tour on the GDMBR this summer. Two big advantages I see are that it can be completely disengaged for zero drag and it is way lighter than a dynohub. There is a fairly extensive thread on them (I think in the electronics sub-forum) that I read before making my purchase.