The energy supplied to the magnetosphere by the solar wind has been estimated by computing the incremental energy build up in a simple magnetotail model as a result of magnetic flux transfer from the dayside via reconnection. A method utilizing the auroral AL index as a quantitative measure of the flux transfer process and, hence, the magnetospheric energy supply is presented. In this way the available AL records are examined for the purpose of determining the variation in energy input from the solar wind and its relationship to interplanetary conditions during 9 years of solar cycle 20 (1966--1974). The principal feature discerned on shorter time scales was a large modulation of the input energy with sector structure, while in the annual averages enhancements were observed in 1968, 1973, and 1974 of 19%, 33%, and 60% over the 1966--1972 mean of 3¿1025 ergs/yr. Interpreting these results as being due to variations in the time history of the dawn-dusk component of the interplanetary electric field in magnetospheric coordinates, Bz-V, leads to the conclusion that while on shorter intervals the energy supply is dominated by variations in Bz-, associated with both the IMF and the orientation of the terrestrial dipole, on the time scale of years Bz- changes are small compared with the solar cycle variation in V which clearly produced the increase in 1973--1974 as has been concluded in other studies using techniques different from the one employed here.