Another approach would be to reduce power consumption by fitting LED stop/tail, pilot and instrument bulbs (either positive or negative earth depending on polarity)?

http://www.norbsa02.freeuk.com/goffyleds.htm

Using a bright (20W) pilot lamp for daytime riding as an alternative to the main headlamp can also save power although this may require a minor change to the wiring so the pilot and main headlamp bulbs are not on at the same time.