If the ladder is vertical, then the work done in ascending equals his increase in potential energy.
For a man of mass (m) climbing height (h), the PE is (mgh), where g is the acceleration due to gravity.
If he does this in time t, his average power (rate of working) is:
mgh / t
= 80 * 9.81 * 6 / 12
= 392.4 W.
If the ladder is not vertical but inclined at an angle the horizontal, the power is mgh sin(a) / t.