In the old days there was a good old measure of power - it's called Watts!

And try to understand one other thing when shopping for a light - if it is not using at least a 6V or 7.2V power source then it should be considered either a very short run time or very dim light.

In other word, except for some exotic flashlights you need a 4xAA (6Volt) or Lithium Ion battery source.

Too bad more dealers don't let you "demo" ride lights - in real ride circumstances.