We report a detailed experimental and modeling study of efficiency for dielectric barrier discharge excited lamp containing mixtures of xenon and iodine. A survey of performance as functions of pressure, iodine partial pressure and excitation parameters indicates that the maximum efficiency of conversion from electrical output to UV (253nm) is in the range 1-2% in contrast to previous reports of efficiency > 20% [eg. Zhang and Boyd, J. Appl. Phys 84, 1174 (1998)]. The results are in good agreement with a 1-D rate-equation model of the discharge kinetics, which suggests that direct quenching of the upper states of the Xel* (B-X) transition (253nm) by I₂ limits the output efficiency.