What is reactive power?

What is reactive power?
Reactive power is an odd topic in AC (Alternating Current) powersystems, and it's usually explained with vector mathematics orphase-shift sinewave graphs. However, a non-math verbal explanationis possible.

Note that Reactive power only becomes important when an "electricalload" or a home appliance contains coils or capacitors. If theelectrical load behaves purely as a resistor, (such as a heater orincandescent bulb for example,) then the device consumes "realpower" only. Reactive power and "power factor" can be ignored, andit can be analysed using an AC version of Ohm's law.

Reactive power is simply this: when a coil or capacitor isconnected to an AC power supply, the coil or capacitor storeselectrical energy during one-fourth of an AC cycle. But then duringthe next quarter-cycle, the coil or capacitor dumps all the storedenergy back into the distant AC power supply. Ideal coils andcapacitors consume no electrical energy, yet they create asignificant electric current. This is very different from aresistor which genuinely consumes electrical energy, and where theelectrical energy flows continously in one direction; moving fromsource to load.

In other words, if your electrical appliance contains inductance orcapacitance, then electrical energy will periodically return to thepower plant, and it will flow back and forth across the powerlines. This leads to an extra current in the power lines, a currentwhich heats the power lines, but which isn't used to provide energyto the appliance. The coil or capacitor causes electrical energy tobegin "sloshing" back and forth between the appliance and thedistant AC generator. Electric companies must install heavier wiresto tolerate the excess current, and they will charge extra for this"unused" energy.

This undesired "energy sloshing" effect can be eliminated. If anelectrical load contains both a coil and capacitor, and if theirresonant frequency is adjusted to exactly 60Hz, then the coil andcapacitor like magic will begin to behave like a pure resistor. The"energy sloshing" still occurs, but now it's all happening betweenthe coil and capacitor, and not in the AC power lines. So, if yourappliance contains a large coil induction motor, you can make themotor behave as a pure resistor, and reduce the current in thepower lines by connecting the right value of capacitance across themotor coil.

Why is reactive power so confusing? Well, the math is daunting ifnot entirely obscure. And the concept of "imaginary power" putsmany people off. But this is not the only problem. Unfortunatelymost of us are taught in grade school that an electric current is aflow of energy, and that energy flows back and forth in AC powerlines. This is completely wrong. In fact the energy flowsconstantly forward, going from source to load. It's only thecharges of the metal wires which flow back and forth.

Imagine that we connect a battery to a light bulb. Electric chargesalready present inside the wires will begin to flow in the circle,and then electrical energy moves almost instantly to the lightbulb. The charge flow is circular like a belt, but the energy flowis one-way. Now imagine that we suddenly reverse the connections tothe battery. The voltage and current will reverse... but the energystill flows in the same direction as before. It still goes frombattery to bulb. If we keep reversing the battery connections overand over, we'd have an AC system. So, in an AC system, only thevoltage and current are "alternating," while the electrical energyflows one-way, going from source to load. Where AC resistive loadsare concerned, electrical energy does not "alternate." Tounderstand energy flow in AC systems, it's critically importantthat we understand the difference between charge flow (current,amperes) and energy flow (power, watts.)

What is imaginary power? Simple: it's the unused power which flowsbackwards and forwards in the power lines, going back and forthbetween the load's coil or capacitor and the distant AC generator.If your appliance was a pure capacitor or inductor, then it wouldconsume no electrical energy at all, but instead allthe flowing energy would take the form of "sloshing energy," andwe'd call it "imaginary power." Of course it's not actuallyimaginary. Instead it's reflected by the load.


What is real power? Even more simple: it's the energy flow whichgoes continuously from the AC generator and into the appliance,without any of it returning back to the distant generator.

Finally, what is "apparent" power? It's just the combination of theabove two ideas: it is the continous-forward-moving or "real"energy flow, combined with the sloshing or "imaginary" energy flow.