Originally Posted by
noglider
True, the almost-free 5-or-so watts available from a dynamo might be more valuable for charging. You could actually run more than one dynamo. For example, one could be your front hub and you could use a sidewall dynamo which you flick on when you need to. I don't know how noticeable the overall drag would be. For sure you would notice it. There are even a few dynamos for rear hubs but they are rare and probably not viable in this case.
I think all the dynhubs put out about the same power since they are all built to German lighting standards.
But the USB chargers vary a lot. Wattage depends on the USB charger you are using. Touring, I use a Sinewave Revolution. Near home on exercise rides, I have measured 2.5 watts out of it on ground that was fairly flat. The electric meter I was using did not record time when there was no charge, thus stop lights, stop signs, and other stops do not count in that time. From that I am assuming 2 watts per hour. On hilly terrain where I am much slower, probably cut that average in half with a lot of variation. On the quick downhills you do not make up the power you lost on the slow uphills. If I am using the granny gear on my triple or the low range on my Rohloff when climbing, I assume zero power output.
THere are two chargers out there, Forumslader and Cycle2Charge, both of which claim much higher output than the Sinewave. And there are several other chargers out there too, but to the best of my knowledge only those two claim more output than the Sinewave. There is not a lot of data on comparisons, there might be others too that I am unaware of. The market for these devices is pretty small, so you do not have a lot of comparisons. And very few users actually measure output from the equipment they own.
I have heard from one Forumslader user that he could feel the drag from it when he was riding and producing power. I can't feel the extra drag from my Sinewave when I am using it. A lot of the dynohubs are running efficiency around maybe 50 percent, total drag is roughly double the power output. But the USB devices have very little power loss. So, if I am getting 2.5 watts out of my dynohub, that likely is 5 watts of total drag. A racer that is competing for time might feel that level of drag but I can't, especially on a bike loaded down with camping gear.
I also have a B&M Luxos U, that has a small built in battery so it is hard to get a good measurement of power out of it since you can't measure how much power is going in or out of the internal battery, but I am quite certain that it produces a lot less power than the Sinewave. I think the circuitry in it is likely similar to the Werk.
I know someone that put a dynhub on a trailer for touring, I do not not use a trailer when touring. But Sidewall dyno or Velological, I can't imagine even thinking about using one of them in addition to a dynohub.
5 watts would be nice, but more realistically on a good day you can cut that in half and on many days, halve that again. Steep hilly days, assume zero.