  The energy spectrum of gravitational waves (GWs), which depicts the energy of
GWs per unit volume of space per logarithmic frequency interval normalized to
the critical density of the Universe, is a widely used way for quantifying the
sensitivity of GW detectors and the strength of GWs, since it has the advantage
of having a clear physical meaning. It was found that the energy spectrum of
GWs depends on the gauge when the GWs beyond the linear order perturbations are
considered. We show that this gauge dependence issue originates from the
inappropriate description for the energy of GWs. With the proper description
for the energy of GWs, we give a well-defined energy spectrum of GWs, in which
the gauge issue disappears naturally.
