A Seebeck-effect calorimeter was used to establish that generation of energy, in excess of the electrical energy input, can occur during the electrolysis of D2O. The magnitude of the excess power is measured with respect to the electrolysis of H2O as the baseline. The excess power levels of >60 W/cm3 palladium and excess energies of 74 kJ cannot be understood in terms of recombination of D2 and O2 within the calorimeter, other chemical reactions, or a storage-and-relaxation mechanism.