  Radiative efficiency of a slowly accreting black hole is estimated using a
two-temperature model of accretion. The radiative efficiency depends on the
magnetic field strength near the Schwarzschild radius. For weak magnetic fields
(magnetic energy=equipartition/1000), the low efficiency 0.0001 assumed in some
theoretical models might be achieved. For stronger fields, a significant
fraction of viscous heat is dissipated by electrons and radiated away resulting
in a larger efficiency. At equipartition magnetic fields, we estimate
efficiency = of order 10%.
