That's an important consideration. The morning I hit the idiot walking the bike lane dressed in all black, my battery was lower than I realized. I was running a nice 24v halogen from an EVG bike, and powering it from a 24v tool nicad. Seemed like I could see fine that morning, but clearly not good enough to see a ninja in time. It was the day of the time change, and suddenly pitch dark later in the morning. Hit that doofus going 27 mph.
Now I rarely ride in full dark. I wait for the dawn glow, and run a flashlight style bike light for them to see me, not for me to see. I'm so old now, I don't really go out after dinner anymore.