Well, manufacturers should rate LEDs using both metrics. Wattage being a measure of how much power it uses, or at least it was designed to run at. Generally for LEDs, the higher the wattage, the better, even if you are driving it at a lower voltage. This leads to more efficiency. That said, the cheaper brands usually say things in watts which usually means nothing to how bright it is. Lumens is the actual measurement of how bright the light is, not at just a point but overall. Bottom line, look for lumens in relation to battery life at that lumen level.