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.