Power Factor
Power factor is the ratio of real power — the power that actually does work — to apparent power, the simple product of RMS voltage and RMS current that a meter reading volts and amps separately would report, and the gap between the two exists because voltage and current in an AC circuit don't have to rise and fall in lockstep: an inductive motor winding drags current out of phase with voltage, and a power factor below 1 is the direct numerical signature of exactly how much current is doing no real work at all.
The phase angle between voltage and current is the entire story, and it has a simple physical cause in reactive loads. A purely resistive load — a heater, an incandescent bulb — draws current that rises and falls in perfect lockstep with voltage, giving a power factor of exactly 1; an inductive load like a motor winding or a transformer stores and releases energy in its magnetic field every cycle, which delays the current's rise and fall relative to voltage by some angle theta, and the cosine of that angle is the power factor — multiplying real power (watts) by that cosine recovers the fraction of apparent power (volt-amps) that's actually doing useful work, with the rest just sloshing reactive energy back and forth between the source and the load's magnetic or electric field without being consumed.
Low power factor costs real money even though the reactive current technically does zero net work, because the infrastructure still has to carry it. A kilowatt-hour meter bills for real power only, but the wires, transformers, and generators feeding a low-power-factor load still have to be sized for the full apparent current — a motor drawing 100 amps at 0.7 power factor demands the same wire gauge and transformer capacity as a resistive load drawing 100 amps at unity power factor, even though it's only billing for 70% as many real watts, which is exactly why utilities impose power factor penalty charges on industrial customers running large banks of motors or transformers: the utility's copper and iron are being used by current that never shows up on the customer's energy bill.
Power factor correction capacitors work by canceling the inductive reactance's phase shift, not by generating any real power at all. A capacitor's current leads voltage by 90 degrees, exactly the opposite direction an inductive load's current lags it, so adding capacitance in parallel with an inductive load lets the capacitor supply the lagging load's reactive current locally instead of making the utility's generator and wiring supply it from across the grid — correctly sized capacitor banks can push a factory's power factor from a penalized 0.7 or 0.8 up toward 0.95 or better without changing how much real work the motors actually do, which is the entire reason power factor correction equipment exists as a standard line item in industrial electrical design.
Modern nonlinear loads introduce a second, genuinely different kind of power factor loss that capacitors alone cannot fix: harmonic distortion. Switching power supplies, variable-frequency motor drives, and LED driver circuits draw current in sharp, non-sinusoidal pulses rather than a clean sine wave, which spreads the current's energy across harmonic frequencies beyond the fundamental — true power factor in this case splits into a displacement factor (the phase-angle effect simple capacitors correct) multiplied by a distortion factor (driven by how non-sinusoidal the current waveform is), and no amount of reactive-power capacitor correction fixes the distortion term; that requires active power factor correction circuitry built into the load itself, which is why modern switching supplies above a certain power rating include dedicated PFC stages rather than relying on passive capacitors downstream.
Read power factor through a *phase-gap-costs-capacity* lens rather than a *billing-trick* lens: the number itself is just the cosine of how far current lags or leads voltage, but that lag forces the entire electrical supply chain — wires, transformers, generators — to carry more current than the real power delivered would otherwise require, which is exactly why correcting it with capacitors or active PFC circuitry pays for itself even though no additional real work gets done.