Power Factor (PF) is the average of any integer number of cycles for the ratio of real to apparent. Apparent is the square root sum of real and reactive squared, also known as the orthogonal vector sum.
But efficiency is only about the ratio of useful real power to that which is supplied. Changing the reactance internal to a circuit can create effects to change efficiency. But to include reactive power in efficiency would lead to false estimates of power lost in heat = power dissipated. This is important when considering the cooling design.
Low PF is considered poor because the currents that just cycle in stored reactance power cause losses in the distribution to the load and transformer saturation margins or available real power in large transformers will be reduced measured as a utilization factor. Low pF is also contributed by non-linear parts (diodes) that interrupt the linear current. It may also occur from complex loads such as;
- high-frequency (HF) devices,
- over-excited synchronous motors, (OESM)
- resonant circuits,
- unbalanced 3-phase loads,
- dynamic loads,
- intermittent loads like welders, which are also HF, even if controlled by some current limit
Since low PF can be corrected by C banks, OESM, and harmonic filters in distribution, the maximum power margin loss is never included in efficiency calculations. Residential meters are designed to measure real power by the motor torque effect of eddy currents in a rotating aluminum disk on a magnetic "air-bearing" or by the real V(t)*Re{I(t)} = Pd calculations.
Thus, reactive power is not considered during the calculation of the efficiency of an SMPS.
VA Reactive or VAR power in large industrial sites can be measured independently and billed when appropriate.