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Work is still done when the user lifts up the 10-15 Kg bag of sand. That has to be counted. It is basically 'free', but from an engineering standpoint, it is some 20-30 "meter-Kg-seconds" of input power, and I assure you the mechanical generator in the Gravity Light is nowhere as efficient (30-50%?) as the plug-in-wall converter (80-95%). It is the world's most efficient, if the only consideration is 'zero utility electricity' consumption. It's like the old cuckoo clock. has to be wound. It's still a great and very-needed product and I hope it succeeds.
Posted by opcom