= Solution
The <specific orbital energy> released in a <black-hole accretion> flow reaching an inner radius $r_{\rm in}$ is of order $GM/r_{\rm in}$. For a circular <Keplerian orbit>, the binding energy is more precisely $GM/(2r_{\rm in})$, with the other half of the potential-energy decrease retained as orbital <kinetic energy>. Multiplying by the <mass accretion rate> gives the <accretion luminosity> scale
$$
L_{\rm acc}\sim\frac{GM\dot M}{r_{\rm in}}.
$$
For an inner radius of order the <Schwarzschild radius>, $r_{\rm in}\sim2GM/c^2$, this becomes
$$
\boxed{L_{\rm acc}\sim\dot M c^2.}
$$
This is an energy-release scaling, not unit <radiative efficiency of black-hole accretion>. The actual inner orbit, stresses and advected energy determine the coefficient and the emitted luminosity; the <innermost stable circular orbit> of a <Schwarzschild black hole> lies outside its horizon.
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