# Watts the difference?



## derekleffew (Aug 4, 2011)

STUDENTS ONLY!

Inspired by http://www.controlbooth.com/forums/...thing-similar-opinions-please.html#post223184 .

With all due respect to West Virginia, what's the difference between W (watts) and VA (volt amps)?


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## shiben (Aug 4, 2011)

VA is for AC circuits, Watts for DC circuits? I feel like it has to do with VA uses RMS Voltage and Amperage, while Watts is nominal? Both measure power, measuring energy expended over time...


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## ScottT (Aug 4, 2011)

Both VA and watts are the measure of energy consumption rate.


The difference comes when you look at different types of currents: AC versus DC.


VA is the *apparent power* of an AC circuit. It is equal to the RMS voltage times RMS current.
Watt is the rate work is done when one amp of current something something something...

Brain fart for the rest of it, someone want to finish it up for me?


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## tolienbosheit (Aug 4, 2011)

ScottT said:


> VA is the *apparent power* of an AC circuit. It is equal to the RMS voltage times RMS current.
> Watt is the rate work is done when one amp of current something something something...



A Watt is the rate work is done when on amp of current flows through an electrical potential difference of one volt, I believe.

~Aaron


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## shiben (Aug 4, 2011)

tolienbosheit said:


> A Watt is the rate work is done when on amp of current flows through an electrical potential difference of one volt, I believe.
> 
> ~Aaron


 
Or Joules/seconds. Watt is actually the SI unit for power, and measures energy use, I think. But in terms of E&M, that sounds right?


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## zmb (Aug 4, 2011)

Volt Amps and Watts can be equal, but usually differ depending on the AC waveform?


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## headcrab (Aug 5, 2011)

Volt-amps are literally (volts x amps). There is no consideration for power factor. Only valid for AC loads.
Watts include power factor. It is calculated as (volts x amps x pf).
For AC loads, power has to be generated to supply the VA load, not the wattage load.

With DC current, there is no phase angle between voltage and current, so power is measured in watts.


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## shiben (Aug 5, 2011)

headcrab said:


> Volt-amps are literally (volts x amps). There is no consideration for power factor. Only valid for AC loads.
> Watts include power factor. It is calculated as (volts x amps x pf).
> For AC loads, power has to be generated to supply the VA load, not the wattage load.
> 
> With DC current, there is no phase angle between voltage and current, so power is measured in watts.


 
Just going to say, DC current is redundant. Probably should be DC circuits.


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## headcrab (Aug 5, 2011)

shiben said:


> Just going to say, DC current is redundant. Probably should be DC circuits.


 
(smacks forehead) Thanks for reminding me.


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## JD (Aug 14, 2011)

Ok, it has been a week so here's what is going on. 

The only time VA = Watts is when you have a load that has a power factor of 1 (or 100% depending on which system you use.) This would be the case with a pure resistive load where the voltage waveform exactly overlays the current waveform. This is not the case with switching power supplies, induction motors, or HID lamp ballasts, etc.

Using the example of the HID ballast: Uncorrected, the greatest current draw does not occur at the peak of line voltage. In fact, if you look at a 100 watt HID, the average current draw may be higher than 2 amps at 120 volts. It is actually using only 100 watts of power as it is drawing most of it's power at a lower voltage in the waveform, but it's VA = 240 (or more!) It is important to note that this is a separate issue from the startup current, which also must be factored in. 

Because this current draw is very real, the support wiring and distribution must be chosen based on this higher number. 10 such fixtures would continuously draw 20 amps, even though the wattage math would indicate the draw should be 8.33 amps. You can see why this becomes very important when working off of a generator. 

Power factor can be corrected in most cases. The ideal is to try to get it as close to 100% as is practical. When it comes to sources of power, such as a generator, the rating may be given in both Watts and/or VA. The difference between the two is an indicator that the generator is designed to handle the excess current. Since this will not effect the actual work being done by the engine, the VA will be the larger number. 

Some supplies, such as battery backup units for computers, have no expectation of running anything but lower powered factor devices (computers) so they only give the VA rating of the device.


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## ejsandstrom (Aug 27, 2011)

JD said:


> You can see why this becomes very important when working off of a generator.



Most commercial generators are rated at .8PF for this very reason. This is also why you never buy based on the KVA rating of a generator. I have built generators that were 200Kw but 600KVA. 

Power factor correction can be very expensive. 

Harmonics play a role in this as well. You may peak the amp draw at 2x the voltage sine wave but only for the first quarter of the sine wave. 

Again Isoc governing and PMG regulation is needed to correctly address this.


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## JD (Aug 29, 2011)

Advanced Info:

To make matters even more interesting, on HID lamp "reactor" style ballasts (and others), as the magnetic field collapses at the end of the cycle, the coil produces a current flow inverse to the general flow. The object of the ballast is to supply the lamp with a near square wave voltage so the lamp does not drop out as the voltage crosses the 0 volt make. (Square wave minimizes transitional voltages as it spends the lion's share of it's time at the peak value.) Even though this current is being produced by the ballast, the nature of the series circuit means the wiring and upstream equipment must handle this current as well.

To correct the power factor, a capacitor of specific value is chosen to be wired in parallel to the ballast*. This capacitor takes on charge as the voltage ramps up, and discharges as the voltage comes back down, effectively helping to support the greater current draw later in the cycle. The result is the overall draw becomes more sinusoidal in nature, therefore bringing the power factor number closer to 100%

Although correcting the power factor of HID lamps is rather easy, the same can not be said about switch-mode power supplies, which chomp most of their current right at the peak of the waveform! 

(* = Many auto-transformer type ballasts put the cap in series with the lamp. "Reactor" type ballasts put the cap in parallel to the incoming line.)


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## arionbarry (Sep 2, 2011)

I have Googled for you and i think following links will be helpful

Watts vs Volt-Amperes

What is the difference between Volt/Amps (V*A) and Watts???....both are related to the power equation. P=VA? - Yahoo! Answers


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## rajsukul (Mar 12, 2012)

Watt for AC Circuits, the other DC. I think? -R


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## Catwalker (May 22, 2014)

A watt is equivalent to one joule per second. It's a measure of power consumption.


Sent from my iPod touch using Tapatalk


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## Chris15 (May 22, 2014)

That's true, but this thread was last active over 2 years ago...


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