Low power factor often means poor input current quality and inefficiencies, which generates cost burdens on the supplier, the consumer, or both. In AC systems, low power factor often comes from input current waveforms that suffer distortions, which is why several international electrical standards impose strict limits on the amount of harmonic content in a supply current, and why active or passive power factor correction is almost mandatory in some circumstances.
Power Factor in Ideal AC Systems
In sinusoidal AC systems, power factor is just the ratio between the real power an electrical system uses to produce work and the total apparent power, which includes real power and reactive power owing to reactive impedance generated by electrical and magnetic fields produced by loads. Reactive impedance causes the current waveform to lag the voltage waveform by a margin known as the phase shift angle, so we can express power factor as the cosine of the phase shift angle between voltage and current waveforms and conclude that power factor approaches the maximum value of one as the phase shift angle between the current and voltage waveforms decreases towards zero.
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https://blog.st.com/converter-digital-pfc-controllers/
