Someone wrote this in the comments over there at Engadget:
" Urea has an enthalpy of formation of -45.9KJ/mol. So it takes +45.9KJ/mol to split it into constitute elements, including 2H2. This hydrogen can be used to form two moles of H2O: 2 x dH(f) H20 = 2 x -241.8kJ/mol = -483.KJThis net process produces -483.KJ/mol + 45.9KJ/mol = -437.7KJ/mol urea
Each litter of Urine contains about 9.3g of urea, and urea weighs 60.06g/mol. So each litter contains 0.155mol of urea. So a litter can theoretically produce 0.155mol/l * 437.7KJ/mol = -67.8 KJ/L
Over 6 hours this is 3.14 watts. And assuming I haven't done anything stupid, if the process is 100% efficient that is the maximum possible energy that can be extracted from the system. In real life the process is probably very inefficient. "