Originally Posted by
hamster
I have a 300 lumen Cygolite Metro and it is adequate for commuting (and, in my experience, outshines 90% of lights that I see at double centuries). It is plenty bright, but I wouldn't mind having extra lumens in the form of a wider beam.
There are two drawbacks of going above 300. Standard headlight design consists of a LED bulb and a rechargeable lithium battery. Good LEDs don't differ much from one another and give to the tune of 60 lumen per watt. Lithium batteries are 120-150 Wh/kg. That means that a 300 lumen headlight with 3 hour life will require a 15 Wh battery that weighs at least 100 g. (And then there's the LED itself, casing, lens, etc.) Most lights are charged through a USB port, which, by design, can't deliver more than 2.5 W, and it would take 6 hours to charge a fully depleted light.
If you go from 300 to 600 at 3 hour life, that gets you to 200 g and 12 hours to charge. Any further, and you end up with a light that's heavy as a brick and requires a dedicated charger because USB does not cut it any more.
Most lights are not charged through a usb port. I have 3 bike lights plus a box full of blinkies, none of them are charged through usb ports. mini usb has a limited life cycle before it conks out. (I also go out of my way not to ever buy lights using usb charging only, because that's just dumb). Also, usb does deliver more than 2.5w, just not through usb 2.0, usb 3.0 delivers more than 2.5w, usb dedicated wall chargers for tablets and newer smartphones deliver around 10w or more.
LED differs a lot. The newer leds are 150 lumens per watt. My newest light can be powered at 600 lumens for 5 hours on a 200 gram battery, which weights not a whole lot. That battery takes 4 hours to charge because it's not usb.
If you need 300+ lumen for more than 5 hours, you should be looking at dynamo lights, not battery lights. For folks who want to avoid charging their bike lights everyday or every second day, dynamo lights is the way to go.