riffraff, I have taken out the instructions to review. According to the instructions that came with the light and usb, the USB is rated for 5 V up to 1 amp. The instructions for the dynamo state that the unit puts out 3 watts. (I'm not an electrician and don't really understand watts, amps, etc.) It is not recommended to charge and have the headlight on at the same time, which is probably what I was doing thinking about this retrospectively. It is very distinct when the iPhone gets power to charge, as there is an audible signal. Once the cache battery is fully charged the audible signal indicates the charge mode, and that happened sometime after 18 km/hr, so I attempted to stay at minimum 20 km/hr. On tour my daily average speed was always around 23 kmph, so really, I was just barely getting the job done-although it doesn't matter if I averaged 30 km/hr, the dynamo will only produce 3 watts.
The system only barely met my needs on tour. I wasn't blown away with the performance-BUT as I say I was pushing the limits attempting to charge, and operate head and tail light.