  The effective energy of a superconductor $E_{eff}(T)$ at temperature $T$ is
defined as the difference between the total energy at temperature $T$ and the
total energy at 0~K. We call the energy of the condensate, ${\mathcal E}_c$,
the difference between $E_{eff}$ and the sum of the quasiparticle energies
$E_{qp}$. ${\mathcal E}_c$, $E_{qp}$, as well as the BCS quasiparticle energy
$\epsilon$ are positive and depend on the gap energy $\Delta$, which, in turn,
depends on the populations of the quasiparticle states (equivalently, they
depend on $T$). So from the energy point of view the superconductor is a Fermi
liquid of non-ideal quasiparticles.
  We show that the choice of quasiparticles is not unique, but there is an
infinite range of possibilities. Some of these possibilities have been explored
in the context of the fractional exclusion statistics (FES), which is a general
method of describing interacting particle systems as ideal gases. We apply FES
here and transform the Fermi liquid of BCS excitations into an ideal gas by
redefining the quasiparticle energies. The new FES quasiparticles exhibit the
same energy gap as the BCS quasiparticles, but a different DOS, which is finite
at any quasiparticle energy.
  We also discuss the effect of the remnant electron-electron interaction
(electron-electron interaction beyond the BCS pairing model) and show that this
can stabilize the BCS condensate, increasing the critical temperature.
