The light pattern is a major factor. You could have a very bright light that is putting a lot of light above the horizon or in other areas where you really don't need much light. Or with a carefully designed reflector and lens you could put all the available light where you want it most, consuming a lot less power.
I am pretty happy with the light pattern out of my B&M Luxos U or my AXA Luxx 70 Plus, both are dyno powered and spread the light out well enough to the sides without wasting a lot of light up higher above the horizon. And both of those lights turn on additional LEDs to put more light on the ground closer to the rider at lower speeds. There are many other lights with similar characteristics, I only mention these two because I own and use them.
On a whim, I bought a $5 USD very bright LED light that has no battery, powered by USB on Ebay, shipped from China. I think it will draw about two to three times as much power as my dynohub can produce at the higher power setting, but it has a beam much like a flashlight. It produces a lot more light than my good lights, but does not do a very good job of putting the light where I want it.
Originally Posted by
Abu Mahendra
i carry a 10,000mAH powerbank, roughly the size and weight of a phablet. i wonder, how long would one have to pedal a dynamo hub to have it produce 10,000mAH of power over 5 volts...
Your powerbank is about 50 watt hours of capacity, I usually get about 2.5 to 3 watts out of my USB port on my bike at typical speeds on the flats, more downhill and less uphill. If you used 2.5 watts as typical then it would take 20 hours of rolling to produce as much as your powerbank.