  An analytical expression for the disc power is derived based on an equivalent
circuit in black hole (BH) magnetosphere with a mapping relation between the
radial coordinate of the disc and that of unknown astrophysical load. It turns
out that this disc power is comparable with two other disc powers derived in
the Poynting flux and hydrodynamic regimes, respectively. In addition, the
relative importance of the disc power relative to the BZ power is discussed. It
is shown that the BZ power is generally dominated by the disc power except some
extreme cases. Furthermore, we show that the disc power derived in our model
can be well fitted with the jet power of M87.
