Originally Posted by
andy_p
If you're doing a long, multi-day ride, you're probably spending less time coasting than at low wheel rpm (where a dynohub can't generate much power). You could also just keep pedalling!
StVZO requires dynamos to provide 6V at 0.5A into a 12 ohm load when riding at 15 km/h which is 9.4 MPH.
Outside the mountains you're not going to have problems running a bicycle GPS. I started and finished rides at 99% on my Garmin Edge 800 until the battery got older - now it finishes at 97%.
Modern cell phones use too much power to keep them charged with the screen on.