The two most important metrics to look for when buying a flashlight are battery and lumens.
Battery: There are two broad categories of both flashlights and headlamps: rechargeable and battery-powered flashlights. The difference is in how you fuel them. AA or AAA batteries are easy to find and can be replaced quickly. When your light goes out in the dark, you’ll see the awesomeness of separate batteries. That said, rechargeable lights can charge the same power bank you’re charging your device, and today, with USB-C, many lights can recharge in less than an hour.
I used to use the battery option and I still recommend a headlamp or flashlight with removable batteries in many cases (your emergency kit for example), but I have to say I found the convenience of a charging port for an everyday carry flashlight or camping headlamp. I personally rely on one Nitecore NU25 in most situations except the long backpacking trips I take with me Petzl Tikka and backup battery pack.
Lumens: Flashlight and headlight companies like to pay a lot of attention to lumens, the measure of a flashlight’s brightness, but the truth is that you don’t always want more lumens. We haven’t tested a flashlight or headlamp that wasn’t bright enough. You’ll find lights here ranging from 300 lumens to 1,000 lumens that will temporarily blind you if you so much as glance at them. For most uses, this is a good range: 300 to 1,000 lumens. For things like reading in a tent at night, you’ll need less light, but any good headlamp will have a variety of brightness levels, usually in a low power mode, around 40 to 60 lumens, which is the level you want to read. Also, look for a strobe mode that will emit light. The strobe function is useful for rescue situations because a flash is more noticeable than a steady light.
The light source for most flashlights and headlights today is LED. Although there are still plenty of traditional light bulbs around, the light emitted to energy used ratio of LED lights is difficult to overcome.
