Gravity Always Wins

Via Slashdot, this is a terribly neat lamp.

I want more details so I can figure out the physics involved. We're given the height of the column, but not the weight of the mass sled, and there's two different figures for the running time (4 hours and 8 hours). Is that enough energy to generate the light output they state?

Some numbers: Single high output LED: 20 mA at ~5V (thus energy use of ~ .1W). They state there's about 10 high output LEDs, so that's 1W. This seems reasonable, as most LED light bulbs currently in use are about .9W or so at the low end. A 1 kg weight at a height of 1.2m (~4 ft) has about 12J of potential energy. That's nowhere near enough to power the LEDs for timescales of an hour. I'm clearly missing something for how this works, or my elementary physics is rusty.

Any ideas? Corrections to my physics are welcome, or other insight into how they can get it to work, etc. :)