Author Topic: KVA vs KW  (Read 25227 times)

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Dan M

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KVA vs KW
« on: January 27, 2005, 04:45:55 PM »


ok, first of all I'm a mechanical engineer (pronounced "sledge hammer jockey by my EE friends).


I'm looking at some big inverters.  When approximating output is there really any difference between a 15 kVA rating and using 15 kW for rough calculations?


Thanks in advance,


-Dan M

« Last Edit: January 27, 2005, 04:45:55 PM by (unknown) »

RatOmeter

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Re: KVA vs KW
« Reply #1 on: January 27, 2005, 11:02:01 AM »
Quoting this link:

http://www.abrconsulting.com/Conversions/elec-con.htm



The only difference between kW and kVA is the power factor.  Once again, the power factor, unless known, is an approximation.  For purposes of our calculations, we use a power factor of .85.  The kVA value is always higher than the value for kW.  


  kW to kVA       kW / .85 = SAME VALUE EXPRESSED IN kVA

  kVA TO kW       kVA * .85 = SAME VALUE EXPRESSED IN kW

« Last Edit: January 27, 2005, 11:02:01 AM by (unknown) »

jimjjnn

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Re: KVA vs KW
« Reply #2 on: January 27, 2005, 11:59:09 AM »
The  only time I can think that this makes a difference is when inverter mfg rate their inverters in kva as rating is higher for kva than kw. I'd rather buy an inverter rated 1.5kw than one rated at 1.5 kva as the kw rating is higher output.


One other thing about kva. I believe it is used more for inductive loads such as motors, and any device with inductors such as coils and transformers.

Still, I'd rather own the 1.5 kw inverter over the1.5 kva inverter as the kw inverter has a higher output in watts.

« Last Edit: January 27, 2005, 11:59:09 AM by (unknown) »

Victor

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Re: KVA vs KW
« Reply #3 on: January 27, 2005, 03:20:31 PM »
Unless a load power factor is specified inverters can only be rated in KVA .

If the inverter is rated in KW there should be a caveat somewhere such as (resistive load) or (powerfactor .85 or better)


 Always remember what the lawyers say: The big print giveth and the small print taketh away.


 In answering your question about comparing inveters I would say most people would call a 4KVA inverter 4KW. Be ware of time constraints on the rating, is it 4KVA continueous,  for 30 min, for 5min, etc. What may be more important is the surge rating as this will determine how large of a motor you can start.


Victor

« Last Edit: January 27, 2005, 03:20:31 PM by (unknown) »

JYL

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Re: KVA vs KW
« Reply #4 on: January 27, 2005, 06:42:32 PM »
A 1KW is equal to 1KVA when you have a resistive load such as an Incandescent lamps, an electric heater without fan, etc...


In this case, 100Watts = 100VA


However, for some CFL with a poor power factor of .5, this is a big deal.  


In this case, 100Watts = 200VA


Unless properly documented, we generally assume a power factor of .7 for Computer, Monitors and good electric motors.  However, cheap electric motor can run has low has .3.  - Cheap Computer Power supply can also have a bad power factor or .5 or .6


A VA is also called "Apparent Power".  This is what an electrical Transformer see.  This power is not necessarily consumed but it is nevertheless needed.


All electric transformer and Inverter are rated in VA unless the load is known in advance.  For example, a 120 VAC to 12VAC transformer for halogen lightning might indicate Watts or VA since this is essentially the same thing.


Otherwise, computer UPS manufacturer use an "educated/Marketing" guest.  For this reason, you can find a computer UPS rated 800VA/500Watts and an others rated 800VA/540Watts.  Sometime ago, I have seen an 800VA/650Watts. None of the above is necessarily better than the others.  The Watts rating is all depending on what the manufacturer consider being a standard Power Factor for a computer load.


Assuming a power factor of .85 is generally adequate for a grid operated transformer.  Otherwise, it might be dangerous to assume it when only a few devices (especially of the same type) are operated from a given Inverter.


For example, operating 400 Watts of CFL might require 800VA or more.   A computer rooms might easily needs a power factor of .6 to operate.  Large motors might have a very poor power factor when starting, needing a lot of VA per watts at that time, etc...  (Here the extreme importance of the surge capacity for an inverter when you operate large motors)


When you mix and match a lot of device:  Capacitive and Inductive, your resulting power factor may actually correct to a point (this is the case for the electrical grid but rarely for an Inverted).  For example, a Leading Power Factor of .5 might cancel a part of a Lagging Power Factor of .4.  So, the exact computation is not easy under this condition.  However, in my experience, this is seldom the case (unless planned) for an Inverter under 100 KVA.


Take note that if you want to purchase an Inverter that produces a true "Sinusoidal wave form", the surge capacity is often very limited.  For example, a 1KVA Inverter might be limited to a short surge of only 1.5 KVA.  Considering this is very important when you have large electric motors (even some washing machine) or, in some cases, very cheap electric motors.  For example, a 1/4HP inductance motors that needs to be fused with a 20AMP/120V breaker to start spinning needs looooots of VA at that time.

« Last Edit: January 27, 2005, 06:42:32 PM by (unknown) »