Dont know enough about electrickery to comment on your desired 15 watts.
My combo is a Son28 disk-braked dynohub with an E-werk
http://www.peterwhitecycles.com/ewerk.asp
and cache battery.
This does my lights, GPS (Edge 800) and cell phone.
I've just built up a second Son28 wheel for an Extra-wheel trailer
http://www.biketrailershop.com/extra...er-p-1377.html
I'm going to be looking for a set up to charge a supposedly 23,000mAh storage battery like:
http://www.youtube.com/watch?v=d-I2z7G8FI4
I currently exploring ways to charge the MP-S23,000
http://www.youtube.com/watch?v=d-I2z7G8FI4
using the trailers Son28.
I may need a step up unit due to an E-werk only reaching 13.3 volts and the battery suggesting it needs 15 volts to charge.
So yeah, the trialer will be for charging my Mackbook Air and camera gear as well as hauling two ten liter Ortlieb water bags.
Thats the plan so far anyway.
Edit:
RE your desired 15 watts
Read something relevant on the Peter While site linked above:
"A small stylus that is housed at the bottom of E-WERK is used for adjusting two turning knobs controlling voltage (2.8 to 13.3 V) and current (0.1 to 1,5 A) quickly and variably for all the usual applications. E-WERK provides power of up to 16 W, and all that with idle losses of only 0.03 W at 30 km/h! At a speed of 15 km/h, E-WERK already charges as fast as a standard mains connected charger."