Where is 99% of the condensation energy of Tl₂Ba₂CuO_y coming from?

Anderson's interlayer tunneling model can account for up to 1% of the condensation energy of Tl_2Ba_2CuO_y. Here we account for the remaining 99%. We predict an in-plane kinetic energy gain of 1 to 3 meV per planar oxygen in the optimally doped material when it goes superconducting. It is suggested that the effect may be most easily detected in underdoped dirty samples.