A self-consistent pseudo-one-dimensional (zero-dimensional with diffusion) plasma model and optical emission spectroscopy an used in tandem to investigate the power coupling efficiency for a pure argon microwave plasma. The self-consistent model is developed by simultaneously solving the Boltzmann equation (for the non-Maxwellian electron energy distribution function), electron number density balance equation, energy balance equation, and the excited state rate equations in a collisional-radiative model. The absolute line emission intensity is utilized to obtain number densities of three argon excited states [4p (7147 Angstrom), 5p (4300 Angstrom), and 5d (6043 Angstrom)] in a 5 Torr, 680 W input power argon discharge. The absolute continuum emission from the plasma was utilized to determine the maximum electron number density. A comparison of the numerical and experimental data indicates that only 2%-5% (10-35 W) of the input power is deposited in the plasma. A control volume heat transfer analysis validates this claim. The experimentally determined energy balance shows that the remaining input power (645-670 W) bypasses the plasma and is dissipated in the reactor cooling systems.