The errors present in electrical measurements at frequencies and impedances relevant to plasma processing in the semiconductor industry are studied. A theoretical bound on calculated delivered power error as a function of measured electrical values is derived. The derivation shows that for constant measurement error, power error is a linear function of load impedance expressed in terms of voltage standing wave ratio. This bound is supported by experimental data taken with both a directional coupler and a voltage- and current-based probe. Linear and nonlinear model based sensing methods are implemented which reduce power error by a factor of 5 over a standard calibration. Published results are cited which indicate that the voltage standing wave ratio of typical plasma processing and experimental regimes is high enough to cause small measurement errors to result in large calculated power errors.