Editing Power factor (section)

==== Lagging, leading and unity power factors ====
Power factor is described as ''leading'' if the current waveform is advanced in phase concerning voltage, or ''lagging'' when the current waveform is behind the voltage waveform. A lagging power factor signifies that the load is inductive, as the load will ''consume'' reactive power. The reactive component <math>Q</math> is positive as reactive power travels through the circuit and is ''consumed'' by the inductive load. A leading power factor signifies that the load is capacitive, as the load ''supplies'' reactive power, and therefore the reactive component <math>Q</math> is negative as reactive power is being supplied to the circuit.

[[File:Lagging-Leading.jpg|frameless|upright=2.66]]

If θ is the [[phase (waves)|phase angle]] between the current and voltage, then the power factor is equal to the [[Trigonometric functions|cosine]] of the angle, <math>\cos\theta</math>:
:<math>|P| = |S| \cos\theta</math>

Since the units are consistent, the power factor is by definition a [[dimensionless number]] between -1 and 1. When the power factor is equal to 0, the energy flow is entirely reactive, and stored energy in the load returns to the source on each cycle. When the power factor is 1, referred to as the ''unity'' power factor, all the energy supplied by the source is consumed by the load. Power factors are usually stated as ''leading'' or ''lagging'' to show the sign of the phase angle. Capacitive loads are leading (current leads voltage), and inductive loads are lagging (current lags voltage).

If a purely resistive load is connected to a power supply, current and voltage will change polarity in step, the power factor will be 1, and the electrical energy flows in a single direction across the network in each cycle. Inductive loads such as induction motors (any type of wound coil) consume reactive power with the current waveform lagging the voltage. Capacitive loads such as capacitor banks or buried cables generate reactive power with the current phase leading the voltage. Both types of loads will absorb energy during part of the AC cycle, which is stored in the device's magnetic or electric field, only to return this energy back to the source during the rest of the cycle.

For example, to get 1&nbsp;kW of real power, if the power factor is unity, 1&nbsp;kVA of apparent power needs to be transferred (1&nbsp;kW ÷ 1 = 1&nbsp;kVA). At low values of power factor, more apparent power needs to be transferred to get the same real power. To get 1&nbsp;kW of real power at 0.2 power factor, 5&nbsp;kVA of apparent power needs to be transferred (1&nbsp;kW ÷ 0.2 = 5&nbsp;kVA). This apparent power must be produced and transmitted to the load and is subject to losses in the production and transmission processes.

Electrical loads consuming [[AC power|alternating current power]] consume both real power and reactive power. The vector sum of real and reactive power is the complex power, and its magnitude is the apparent power. The presence of reactive power causes the real power to be less than the apparent power, and so, the electric load has a power factor of less than 1.

A negative power factor (0 to −1) can result from returning active power to the source, such as in the case of a building fitted with solar panels when surplus power is fed back into the supply.<ref>{{Citation | title = On the resistance and electromotive forces of the electric arc |first=W. | last = Duddell | journal = Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=203 |issue=359–371 |doi=10.1098/rsta.1904.0022  | pages = 512–15 | year = 1901 | quote = The fact that the solid arc has, at low frequencies, a negative power factor, indicates that the arc is supplying power to the alternator…| doi-access = free }}</ref><ref>{{Citation |title=Analysis of some measurement issues in bushing power factor tests in the field |first=S. |last=Zhang |journal= IEEE Transactions on Power Delivery|volume=21 |issue=3 |pages=1350–56 |date=July 2006 |quote=…(the measurement) gives both negative power factor and negative resistive current (power loss) |doi=10.1109/tpwrd.2006.874616|s2cid=39895367 }}</ref><ref>{{Citation | title = Performance of Grid-Connected Induction Generator under Naturally Commutated AC Voltage Controller |first=A. F. |last=Almarshoud |display-authors=etal |journal=Electric Power Components and Systems |volume=32 |issue=7 |pages=691–700 |year=2004 |quote=Accordingly, the generator will consume active power from the grid, which leads to negative power factor.|doi=10.1080/15325000490461064 |s2cid=110279940 }}</ref>