How does the human body use energy?
You can break down the energy expenditure of the human body in terms of the different organs which use it up: this is done in this biochemistry book for instance, http://www.ncbi.nlm.nih.gov/books/NBK22436/

You can also look at the detailed expenditure of each cell: although the amounts will vary according to cell type (neuron vs. muscle cell, say!), the "budget items" are the same. See this thread: Breakdown of energy expenditure at the level of a single cell