  We obtain an energy inequality on null surfaces $u=const$ in the Bondi-Sachs
formalism. We show that for a sufficiently regular event horizon $H$ there is
an affine radial coordinate which is constant on $H$. Then the energy
inequality can be prolongated to the horizon giving an estimation which is
closely related to the Penrose inequality. We test it for the Kerr solution
written in the Fletcher-Lun coordinates.
