LEDs do not use 3.0-3.6 volts, the range is far larger due to different colors being available. In a LED flashlight it will likely range from 1.9 (red) to 3.6 (blue, and thus white as well). Lower voltages are used for IR leds (and perhaps higher for UV leds?) but that is an exception. FjVwxII1 11:04, 30 January 2006 (UTC)
LED's may not use less power to run. They could use more.
Which uses less power to run, a 5w LED bulb, or a 5w incandescent bulb? Most may rush to say, "The LED uses less power." How is that? They are both 5 watts! It's like asking which is heavier, one ton of bricks or one ton of feathers? They are both one ton. However, keep reading.
Not all LED flashlights are going to use less power and give more light. That depends on the bulb. The first Nichia 5mm white LED's do consume a lot less power, but are not great givers of light. Hence the reason why you always see them in clusters. The newer 10mm give off much more light, but I am uncertain if they consume the same relative power.
A growing trend among "flashaholics" (people who love, modify, and collect flashlights) are the high powered LED models. Your standard 3 D-cell Maglite with incandescent bulb is about 24 lumens of light. If you used one, you have an idea of the light volume it generates. The 4.5v bulb probably runs about 1w.
Take that same 3 D-cell Maglite and toss in a Diamond brand 1w LED conversion bulb. Now, you may see about 30 lumens of much whiter light, and the power drain is certainly less, so your batteries last longer. The LED converts most of that energy into light, while the incandescent converts most into heat. Thus the incandescent bulb works harder to get that 24 lumens, where as the LED works less for more light at 30 lumens.
Now, you could also install a 3w LED bulb and may see 50 or 60 lumens of light. But it is sucking more battery juice, thus you expect the battery will not last longer, and may have run times closer to that 1w incandescent bulb. A 3W LED bulb reportedly gives up to 20 hours of usable light. I doubt an incandescent lasts that long, even with 18 amp D-cells.
Therefore, if you are buying LED array flashlights, they are most likely 1W conversions that provide whiter light with less energy consumed. Luxeon brand bulbs are single LED bulbs in 1w, 3w, and 5w versions. These single bulbs are actually better than the array of 5mm LED lights.
But if you are buying a high powered Luxeon 3w or 5w flashlight, expect less battery life. The trend is to put these bulbs in lights that use AA, or lithium CR123 cells. Neither of those cells have the long life of a D cell, or even a C cell. People get VERY bright light at the cost of run time. Apparently, people want them small and bright.