Power Calculator for Generators: Convert kVA to kW, kW to kVA, kW to HP

Submitted: Thursday, Jun 21, 2012 at 15:34
ThreadID: 96407 Views:4550 Replies:1 FollowUps:0
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A few days ago there was a post Re Generators and what size etc was good for one's purpose. In view of this I decided to have a look around the net as to what was available re 'small' generators and the cost of same.

It soon became glaringly obvious that the Manufacturer's labelling and/or Sellers marketing blurb as to exactly what the subject generator's output/s were, was in a lot cases confusing to say the least.

Some were labelled in Kva's and sold Kw's, others labelled in peak output available under start up load and then on reading the Spec Sheet you find that it reverts to much less after start up etc etc.

Also for those not aware of the difference between Kva's and Kw's and assuming that they are identical, the probability of ending up with a product not suited for their intened purpose was a good possibility.

In view of this I located a site on the net that has very useful info on how calculate the size generator that you require in Kw's and then convert same to Kva's by using the very user friendly conversion calculators on the said site.

Calculator Site:Electriacal Power Calculators

Whilst this is a commercial site, as it is in the USA it should be obvious to all and sundry that I have no other interest in it other than the above mentioned calculator .

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Reply By: Nomadic Navara - Friday, Jun 22, 2012 at 15:58

Friday, Jun 22, 2012 at 15:58
It' a bit hard to explain the reason for the term KVA. We all know about the sine wave form of AC voltage and current. In a purely resistive load (a so called linear circuit) current will flow in proportion to the voltage at any instant. This creates a current flow in the same sinusoidal wave shape as the voltage. The two wave forms occur at the same time. This is referred to as being in phase with each other.

The big problem comes when when we have things like electric motors and some fluorescent lights as our load. They are not pure resistors. The load they present to the generator (or grid) has a reactive component associated with the purely resistive component. The resultant current drawn from the supply results in a waveform that does not occur at the same time as the voltage waveform. This is referred to as the two waves are out of phase..

When we have the two waves out of phase the current drawn from the supply is greater than a purely resistive load consuming the same power in watts. When this happens the grid or your generator has got to be able to supply this greater current load even though it is not supplying as much power as you would expect by calculating that power by multiplying the voltage by the current. This is often referred to as the apparent load (which is always greater than the actual load that would measured by your meter on the power board.)

The displacement of the current from being in phase with voltage waveform is the power factor of your device, you will often see it specified on electric motors.

This is a simple explanation of why you need a generator capable of driving larger resistive loads than the rating given for such devices as air conditioners. The term KVA is how some of these devices are specified. A fuller explanation is given (complete with waveforms) at - http://en.wikipedia.org/wiki/Power_factor

To add to the complexity of driving appliances with generators, we have things like battery chargers that really mess up the waveform from generators. These have switching devices in their inputs (like thyristors) that only draw current for parts of the waveform, not the whole part of a half cycle. I can predict what the motors will do to a generator but I would not hazard a guess what one of those battery chargers will do. They are often best run in conjunction with a resistive load like an incandescent light globe.
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