Shimano hubs aren't particularly efficient on or off. If you want to charge something, you're better off with an SP or SoN. It seems like test results differ on which of those is better. Everybody gets about the same answer with SoN, some people measure the SP as being more efficient. I have come to the conclusion that I should buy SoN. For one thing, the price difference is less between disc through axle. But I use my dynohubs long enough that $100 isn't worth worrying about. OTOH, Shimano hubs seem to come my way for very little money, that's why I have so many of them.