  Analytical expression for energy of eigen-modes in magnetohydrodynamic flows
of ideal fluids is obtained. It is shown that the energy of unstable modes is
zero, while the energy of stable oscillatory modes (waves) can assume both
positive and negative values. Negative energy waves always correspond to
non-symmetric eigen-modes -- modes that have a component of wave-vector along
the equilibrium velocity. These results suggest that all non-symmetric
instabilities in ideal MHD systems with flows are associated with coupling of
positive and negative energy waves. As an example the energy of eigen-modes is
calculated for incompressible conducting fluid rotating in axial magnetic
field.
