kVA vs kW
Have you ever noticed that with every
appliance or piece of electrical machinery that you avail, they will
always indicate their respective power ratings?
You will notice that some electrical
equipment express their power ratings in kW, or kilowatts; and some are
expressed in kVA, or kilo Volt Amperes. Both values express power, but they are actually different.
kVA is known as the ‘apparent power’ of
a particular circuit or electrical system. In direct current circuits,
kVA is equal to kW, because voltage and current do not get out of
phase. However, ‘apparent power’ and ‘real power’ (which is expressed
as kW) may differ in alternating current circuits. kW is simply the
amount of actual power that does valid work. It should be noted that
only fraction of kVA is accessible to do work, and the rest is an excess
in the current.
Solving for the kW (real power)
requires another variable called the Power Factor (PF). That so-called
Power Factor is a nebulous value that can vary for every appliance or
electrical device. In essence, the value of the Power Factor is either
given in a percentage, or 0 to 1, wherein 100 percent (or 1) is
considered as unity. The closer the Power Factor is to unity, the more
efficient a particular device is with its use of electricity.
Unity is practically present in DC
circuits, which creates no difference between the kVA and kW. A device
uses less kW when the voltage is out of phase with the current. At the
same time, the Power Factor naturally lowers in the process. Power
Factor will either be leading or lagging, depending on which way the
load shifts the phase of the current with respect to the phase of the
voltage.
The relationship between the three (kVA, kW, and Power Factor) is mathematically described as:
kW = kVA x Power Factor; kVA = kW / Power Factor; Power Factor = kW / kVA
In DC circuits, the power factor is mathematically inconsequential, because it is in unity. Therefore:
kW = kVA = Volts x Current x 1 = Volts x Current
Summary:
1. kVA is known as the ‘apparent power’, while kW refers to the actual, or real power.
2. kW is the amount of power capable of doing work, while only a portion of kVA is available to do work.
3. kW is kilowatts, while kVA is kilo Volts Amperes.
4. kVA is equal to kW in DC circuits because the voltage and current are not out of phase (unity).
5. However, in AC circuits, voltage and
current may get out of phase. Therefore, kW and kVA will differ
depending on the Power Factor, or how much leading or lagging occurs.
