A method for converting heat from a heat source to mechanical energy is provided. The method comprises heating a working fluid E-1,3,3,3-tetrafluoropropene and at least one compound selected from 1,1,1,2-tetrafluoroethane and 1,1,2,2-tetrafluoroethane using heat supplied from the heat source; and expanding the heated working fluid to lower the pressure of the working fluid and generate mechanical energy as the pressure of the working fluid is lowered. Additionally, a power cycle apparatus containing a working fluid to convert heat to mechanical energy is provided. The apparatus contains a working fluid comprising E-1,3,3,3-tetrafluoropropene and at least one compound selected from 1,1,1,2-tetrafluoroethane and 1,1,2,2-tetrafluoroethane. A working fluid is provided comprising an azeotropic or azeotrope-like combination of E-1,3,3,3-tetrafluoropropene, 1,1,2,2-tetrafluoroethane and 1,1,2,2-tetrafluoroethane.