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Power factor is the ratio of active power to apparent power in an AC circuit. It equals the cosine of the phase difference between voltage and current, ranging from zero to one. For a purely resistive load with in-phase voltage and current, the power factor equals unity, meaning all apparent power converts to useful active power.

Utility companies charge for apparent power, which includes both active and reactive power. A lower power factor means more reactive power is wasted without performing real work, increasing electricity bills. Reducing reactive power to bring the power factor closer to unity maximizes value and lowers costs for consumers.

A leading power factor occurs when current leads voltage, indicating a capacitive load. A lagging power factor occurs when current lags behind voltage, indicating an inductive load. Both represent phase differences between voltage and current that reduce the power factor below unity.

Purely resistive loads produce a unity power factor of one, with voltage and current in phase. Purely reactive loads produce a zero power factor with no active power. Real circuits contain combinations of resistive and reactive elements, resulting in power factors between zero and one depending on the load impedance angle.

The power factor angle is the phase difference between voltage and current, equal to the angle of the load impedance. Its cosine yields the power factor value. This angle determines whether the power factor is leading or lagging and directly influences how much active power the circuit delivers.

In this analogy, coffee represents useful active power that performs real work, while foam represents reactive power that does not perform real work but is utilized by certain appliances. The entire glass represents apparent power, which utilities charge for. Reducing foam (reactive power) maximizes the value of what you pay for.

In purely reactive circuits, the phase difference between voltage and current is 90 degrees, resulting in a power factor of zero. This means active power is zero, and all power is reactive. No useful work is performed, though reactive power is essential for maintaining voltage levels in power systems.