Remember the Bicycle Generators in Soylent Green?

A green lighting system:

What could be better for providing light in a poor country with no electricity [for lighting other] than a cheap kerosene lamp? A group of renewable entrepreneurs think they have found the answer – meet Gravity Light, a third world LED lighting system powered by lifting a bag containing 12Kg (27lb) of rocks every 20 minutes.

So every 20 minutes, some poor person has to lift 27 lbs of rock to have light.  So much better (if you live in a First World penthouse) than allowing their country to join at least the Second World.  What is the efficiency of the corn to human energy conversion?