It can be seen that the input voltage is only slightly distorted by the very non-linear load presented by the capacitor input filter. The sinusoidal input is maintained because the line input resistance is very low. The input current however, is very distorted and discontinuous, but superficially would appear to be a part sine wave in phase with the voltage. This leads to a common error: The product V_{in} _{(rms)} x I_{in} _{(rms)} is assumed to give input power. This is not so! The product is the input volt ampere product; it must be multiplied by the power factor (typically 0.6 for a capacitor input filter) to get true power.

The reason for the low power factor is that the non-sinusoidal cur- rent wave form contains a large odd harmonic content, and the phase and amplitude of all harmonics must be included in the measurement.

Apparent power = E(rms) X I(rms)

Active power = Apparent power X power factor

The peak instantaneous input current drawn by the SMPS at switch ON.