They "cooked" cannabis bark into carbon nanosheets and built supercapacitors "on a par with or better than graphene" - the industry gold standard.
Electric cars and power tools could harness this hemp technology, the US researchers say.
They presented their work at the American Chemical Society meeting in San Francisco.
"People ask me: why hemp? I say, why not?" said Dr David Mitlin of Clarkson University, New York, who describes his device in the journal ACS Nano.
"We're making graphene-like materials for a thousandth of the price - and we're doing it with waste.
"The hemp we use is perfectly legal to grow. It has no THC in it at all - so there's no overlap with any recreational activities."