If your generator is a 6-volt 3-watt one, which is the most common, you want a 6-volt, 0.6-watt taillight bulb and a 6-volt, 2.4-watt headlight bulb. When (not if) the headlight bulb fails, it will probably nuke the taillight bulb in a matter of seconds. When (not if) the taillight bulb fails, it will cause your headlight to run extra-bright, but failure is hastened. So you may want to consider a set of LED lights instead of incandescent... they don't burn out.