A method and apparatus for measuring electrical characteristics (e.g. current, voltage, phase, etc.) between a power source and a load at a set of harmonic frequencies to determine information about the load (e.g., load impedance, power dissipation, etc). According to one aspect of the invention, a first circuit detects a set of electrical characteristics (e.g., current, voltage, and/or phase) of a signal between the power source and the load. A second circuit, coupled to the first circuit to receive the set of electrical characteristics, provides data representing the set of electrical characteristics at a harmonic frequency associated with the signal. A third circuit, coupled to the second circuit, receives the data and determines information about the load (e.g., impedance, power dissipation, discharge current, etc.) at the harmonic frequency. The information could be used in any number of ways, such as controlling an impedance matching network that may be used in conjunction with the power source and the load, identifying/monitoring a load condition(s), etc.