Their claim is a 20 pound weight raised 6 feet (or so) in the air will generate light for 30 minutes:
20 lbf * 6 ft / 30 min = ~90 mW
For reference, the LED indicators on your keyboard use about 15 mW each. So that's about two keyboards worth of light.
However maybe I'm being stingy -- because of persistence of vision (a characteristic of the human eye), you can run LEDs (or any light) on a low duty cycle and still produce the same apparent brightness. So if we say they run it at a 10% duty cycle (I'm not sure how accurate this estimate is), they might be able to get closer to 1 W of LED light, which is enough for reading.
The apparent brightness of a rapidly flashing LED isn't anywhere near the peak brightness, you get roughly the same brightness per power emitted regardless of pattern. You'd probably get a lot more efficiency by just using a non-white LED and picking a color to maximize human sensitivity vs. power draw.
Their claim is a 20 pound weight raised 6 feet (or so) in the air will generate light for 30 minutes:
20 lbf * 6 ft / 30 min = ~90 mW
For reference, the LED indicators on your keyboard use about 15 mW each. So that's about two keyboards worth of light.
However maybe I'm being stingy -- because of persistence of vision (a characteristic of the human eye), you can run LEDs (or any light) on a low duty cycle and still produce the same apparent brightness. So if we say they run it at a 10% duty cycle (I'm not sure how accurate this estimate is), they might be able to get closer to 1 W of LED light, which is enough for reading.