We examined 11 cases when the interplanetary magnetic field (IMF) was intensely northward (>10 nT) for long durations of time (>3 hours), to quantitatively determine an upper limit on the efficiency of solar wind energy injection into the magnetosphere. We have specifically selected these large BN events to minimize the effects of magnetic reconnection. Many of these cases occurred during intervals of high-speed streams when viscous interaction effects might be at a maximum. It is found that the typical efficiency of solar wind energy injection into the magnetosphere is 1.0¿10-3 to 4.0¿10-3, 100 to 30 times less efficient than during periods of intense southward IMFs. Other energy sinks not included in these numbers are discussed. Estimates of their magnitudes are provided. ¿ American Geophysical Union 1995