An accurate evaluation of the noise temperature of noncooled antenna systems is of great importance, particularly if the specifications of the antenna alone are a fewKelvins as in applications for radio astronomy. This evaluation requires an accurate analysis of the radiation efficiency, which we evaluate through the computation of the dissipated power from a method-of-moments (MoM) solution.We examine the numerical accuracy and the mesh sensitivity of the computed efficiency. Moreover we show that the efficiency is numerically smaller than or equal to 100%, contrary to results obtained with several commercial solvers. To describe the antenna loss, we employ the surface impedance of a thin sheet. For arrays of loops and for a tapered-slot antenna (TSA), we compare our efficiency results with those obtained by commercial solvers and by numerical far-field integration for the radiated power.