The Physics Education Research Group at University of Maryland has been studying student learning of quantum mechanics. Our previously reported research shows that student difficulties exist with classical concepts that are prerequisite for learning quantum mechanics. In this talk, we report detailed studies of student difficulties in quantum mechanics arising from confusions with the classical concept of energy. Students are confused on some detailed issues concerning energy and energy diagrams such as the possible value of total classical energy, the meaning of discontinuous potential energy diagrams, and the quantization of energy levels in the quantum case. We have also found new induced confusions on classical issues that are created by students misinterpreting quantum concepts. Proposals for instruction to deal with these issues will be discussed.