The effective backscatter coefficient η is a quantity that must be known with precision so that the proximity effect can be adequately compensated to minimize feature size variations in electron beam lithography. A unique technique to measure η that does not require the precise form of the backscatter dose distribution was employed. This method simply compares the resist response near the center of printed features that are much smaller and much larger than the characteristic range of the long range scatter. This technique was already employed to estimate the backscatter coefficient on Si at 100 keV beam energies. We have extended this measurement to determine η at lower beam energies. Results show that η on Si is 0.38, 0.50, 0.55, and 0.46 for 100, 50, 40, and 20 keV beam energies, respectively. Monte Carlo simulations indicate a trend of decreasing η with increasing beam energy, consistent with the experimental results except at 20 keV.