An expression for the total power PT transferred from the solar wind to an ''open'' magnetopause with a nonzero normal component of the magnetic field, which is identified as a rotational discontinuity. The total power PT consists of (1) the power PEM representing the electromagnetic energy transfer and (2) the power PKE representing the rate of kinetic energy carried by particles penetrating into the magnetosphere. It is found that PEM?VSW BSW&psgr;, PKE?(1/2 MA-1) PEM and PT?1/2MAPEM, where VSW, BSW, and MA are the velocity, magnetic field, and the Alfv¿n--Mach number in the solar wind, respectively, and &PSgr; is the open magnetic flux in the magnetosphere. The Alfv¿n--Mach number of flow at the magnetopause determines the nature of the local energy transfer; the power per unit area transferred from the solar wind to the magnetosphere consists mainly of kinetic energy. The electromagnetic energy rate PEM controls the near-earth magnetospheric activity, whereas the kinetic energy rate PKE(?3--4 PEM) should dominate the dynamics of the distant magnetotail.