Or, using the numbers provided for this actual product which exists, rather than a spherical LED on a frictionless plane, we raise a 12kg mass 1.8m and get a 0.1W LED light for 20 minutes. Do you think THAT's practical?
0.1W LED light is as bright as one of those clip on book reading lights, they only look bright in pictures. It's not quite useless, but you can build a 0.1w for 12 hour solar light with a 12' cord for ~2$ making the whole thing a joke.