The active and apparent powers of two converter circuits with different spectrums of their input and output are calculated. The first circuit is a single phase bridge with pure resistive load. This circuit is linear one, with sinusoidal input and non-sinusoidal output. The main power theories allow obtaining the correct results. The second circuit is a simple series diode resistance connection. This circuit is one with nonlinear input (sinusoidal voltage and non-sinusoidal current) and linear non-sinusoidal output. The main power theories do not allow obtaining the correct results of apparent power calculation. The conclusion drawn is that the main power theories allow obtaining the correct results of active power calculations of linear and non-linear circuits. These theories can't be applied for the apparent power calculations of nonlinear circuits and systems.