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In coherent beam combining (phased arrays), control of the relative amplitudes (powers) of array elements is needed to maximize the beam combining efficiency. Here, expressions are derived for the efficiency in approaches that use interferometric combiners such as diffractive optical elements or fiber couplers. The beam combining efficiency is relatively insensitive to small power errors. In the case of dropped elements, the efficiency is equal to the fraction of operating elements. The beam combining efficiency is equivalent to the Strehl ratio for a tiled aperture.