The minimum energy required to remove an electron from the ground state of a hydrogen atom is:

Ionisation energy is the energy required to take the electron from $n = 1$ to $n = \infty$. $E_\infty - E_1 = 0 - (-13.6) = 13.6\,\text{eV}$. The value $27.2\,\text{eV}$ is the magnitude of the potential energy in the ground state, not the ionisation energy.