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Difference Between kVA and kW

energy-powerkVA 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.


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11 Comments

  1. Nice to read your write up on kVA, kW and PF.
    I have tried to clear s few ambiguities in this area in my following blog
    http://lvnaga.wordpress.com/2010/02/19/electrical-power-and-power-factor/
    I value your comments on the same.
    Nagarajan

  2. How much watts can a 1 KVA offline UPS support?

  3. is the KW and KVA rating will be same for a single phase generator?

  4. thanks fo the info on kW and kVA, was searching for it

  5. dear sir,
    what is the diferance of kva and kw. how to caculate?

  6. Very interesting. I’d always assumed KVA and KW were the same in all circumstances. I consider myself now better educated!!

  7. Note with many thanks.

  8. Thanks the note author

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