  The efficiency of thin disk accretion onto black holes depends on the inner
boundary condition, specifically the torque applied to the disk at the last
stable orbit. This is usually assumed to vanish. I estimate the torque on a
magnetized disk using a steady magnetohydrodynamic inflow model originally
developed by Takahashi et al., 1990. I find that the efficiency epsilon can
depart significantly from the classical thin disk value. In some cases epsilon
> 1, i.e. energy is extracted from the black hole.
