First, bulbs are designed to work at a single, specific voltage. In household terms, a 60 watt bulb running on 120 volts in the U.S. is the same brightness as a 60 watt bulb running on 240 volts in Germany. A higher wattage bulb will be brighter - at either voltage. Here are two examples. 10 0.83 amp 3 hrs.
Coiled cords have three problems.
First, they use about three times the amount of wire as a straight cord. More wire means greater resistance, therefore less voltage to the bulbs. Second, they tend to bounce around when you're riding off-road. We think it's best to velcro the wire to the frame and bars. And third, they typically use 22 gauge wire, which is quite thin and has a lot of resistance - particularly with 6 volt systems.