The total heat balance during the electrolysis of D2O with a palladium cathode is determined by placing the entire hermetically sealed electrolysis system (the electrochemical cell connected with a vessel of varying volume) in an isoperibol calorimeter. Significant excess power density (excess specific rate of heating) is obtained even though a palladium cathode of thin wire (0.05-cm diam) is used, in which case a relatively low value of excess energy is expected. The method and arrangement applied remove the main causes of inaccuracies in determining the excess energy. Thus, the possibilities of using this energy seem to be greater than some researchers are inclined to consider.