  A rotating black hole probably powers ultrarelativistic jets in gamma-ray
bursts, relativistic jets from some active galactic nuclei, and jets from some
black hole x-ray binaries. Prior estimates of the power output of a black hole
have assumed an infinitely thin disk, a magnetic field based upon a slowly
rotating black hole, and have not self-consistently determined the geometry or
magnitude of the magnetic field for a realistic accretion disk. We provide
useful formulae for the total and jet Blandford-Znajek (BZ) power and
efficiency as determined self-consistently from general relativistic
magnetohydrodynamic numerical models. Of all jet mechanisms, we suggest that
only the BZ mechanism is able to produce an ultrarelativistic jet.
