The result of an attempt to express the net energy input to the magnetosphere-ionosphere system in terms of upstream solar wind parameters is prexented. Seven types of electric current system within the magnetosphere are defined and discussed, and it is found that only three of these systematically transfer energy into the magnetosphere from the solar wind. One of these is the magnetotail current system, and the other two form the field-aligned current observed at the northern edge of the auroral oval. In order to calculate energy inpur, reasonable models are developed for each of a number of magnetospheric processes. These include (1) flow through the shock, flow in the magnetosheath, and merging; (2) reconnection and the neutral line in the magnetotail; and (3) Alfven layer shielding of the inner magnetosphere from convective flow, nightside pressure balances, and precipitation, and hence from these the width and conductance of the nightside auroral oval. The use of these models permits the development of explicit equations for the total energy input of the magnetosphere-ionosphere system in terms of upstream solar wind parameters. The interplanetary magnetic field strength and orientation and the solar wind velocity dominate energy input via the nightside plasma sheet. The density of the solar wind is also an important variable for the energy input by the high-latitude current systems.