Methods to determine an instantaneous resistance value of an electric circuit and a measurement system to determine an instantaneous resistance value of an electric circuit are disclosed. Exemplary embodiments of the method measure an in-situ instantaneous voltage of the circuit and an in-situ instantaneous current of the circuit and calculate the instantaneous resistance. Optional temperature measurement can be included in the method and the calculated instantaneous resistance related to the measured temperature. The method can be applied to phase angle fired loads and to zero-cross (time proportioned) loads.