Velocipedio,
I ride to work every morning in the dark. I have a 15W on one bike and a 10W on the other. The 10W is plenty; I can't tell a big difference between the two. Much of my ride is on reasonably well-lit city streets where I often can't even see my light on the ground. Where it is dark, I can see quite well enough to observe bumps, rough spots, trash, etc. in plenty of time to adjust course if necessary.
Ride lit. (Sorry, Pat, I won't do it again. )
Regards,
Raymond