Measurements and analysis of the performance of a thermoacoustic engine driving a dissipative load are presented. The effect of the load can be explained qualitatively using a simple low-amplitude approximation and quantitatively by invoking a more accurate low-amplitude numerical solution. The heater power Q̇ and hot-end temperature TH are found to be simple functions of the load impedance and the unloaded values of Q̇ and TH.