Originally Posted by dfw
Lumens may not be all that useful either because it doesn't tell you where the light is going, it's just a measure of how much total light is being produced by the bulb. It doesn't tell you much about how well the lamp is designed. You could have two lights with the exact same # of lumens and one could be much more effective if it focuses the light where you need it most.
Watts are not bad to go by if you're comparing LED to LED, Halogen to Halogen, etc. Watts are not a good thing to go by if you're comparing different technologies. The best measure is to compare them side by side and see what works best for you.
The throw and beam width are better indicaters of whether the light will be good for riding conditions. Beam width can be narrower for mt bike trails etc.
The Throw is how far the light shines in front of you. You can calculate the number of feet/sec you need by your avg speed and reaction time.
So like at 15mph:
15* 5280/3600 (seconds in an hour) = 22' feet * 3 = 66' (your light should shine about 70' in front of you.