  In this paper, we prove that associated with a sub-static asymptotically flat
manifold endowed with a harmonic potential there is a one-parameter family
$\{F_{\beta}\}$ of functions which are monotone along the level-set flow of the
potential. Such monotonicity holds up to the optimal threshold
$\beta=\frac{n-2}{n-1}$ and allows us to prove a geometric capacitary
inequality where the capacity of the horizon plays the same role as the ADM
mass in the celebrated Riemannian Penrose Inequality.
