  The impact of nonlinear refraction and residual absorption on the achievable
peak and average power in beam-splitter-based coherent beam combination is
analyzed theoretically. While the peak power remains limited only by the
aperture size, a fundamental average power limit is given by the thermo-optical
and thermo-mechanical properties of the beam splitter material and its
coatings. Based on our analysis, 100 kW average power can be obtained with
state-of-the-art optics at maintained high beam quality (M2 {\leq} 1.1) and at
only 2% loss of combining efficiency. This result indicates that the
power-scaling potential of today's beam-splitter-based coherent beam
combination is far from being depleted. A potential scaling route to
megawatt-level average power is discussed for optimized beam splitter geometry.
