The amplitude and phase variability among combiner inputs result in combining loss and degradation in amplifier’s combining efficiency. An analysis on combining efficiency is presented to understand the effect of variability in the combiner inputs and combiner transmission parameters. For symmetrical combiners, combining efficiency depends on the variance of its input voltages and intrinsic loss in combiner itself, whereas combining efficiency depends on mean of its inputs as well. Lower bound on combining efficiency has also been derived. The lower bound presented here offers an improvement over the lower bound reported in earlier works. Knowing the maximum spread in amplifier gain (magnitude and phase) in a production process, it is possible to predict the worst case combining efficiency. The derivations have been validated with the help of system simulation using measured results of power combiner.