Most of the common wattages for bulbs fit into two categories. I use a 10W halogen when I commute in the winter; is that 5-10 watts or is that 10-15 watts? Since it's an SLA system that loses brightness as it discharges, I picked 5-10 watts.
50 watts will blind drivers if it's not aimed carefully, and I shudder to think how quickly it'll burn through batteries.
