Episode 19 – Electrical Power (Wattage and Volt-Amps)

Electrical power is often expressed in two units, Watts and VA.  Wattage and Volt-Amps rather, get misused because many people do not understand the differences in them and when they should be used.  In this episode I break down the differences in the two terms and how they relate to one-another in the electrical industry.

Wattage (Watts)

Most people use the word wattage or watts on a pretty regular basis when talking about power consumed in an electrical circuit.  If we have a 75-watt light bulb, we might say that this bulb is “burning” at 75-watts.  This is because this bulb is rated at this wattage based on the materials inside of it.  There is a filament (coil of wire) inside the bulb that has a very specific resistance.  When connecting 120-volts across this filament a certain amount of heat and light is produced because 0.83 amps of current are flowing through the filament. 

The heat and light coming from the filament is technically waste, or “power loss” – however it’s not REALLY lost.  The light, and sometimes heat, is a wanted effect of running current through the high-resistance filament.  In fact, that is the entire purpose of it’s design.  We want that filament to be crammed with current so much that it starts to glow red-hot.  This light bulb is consuming 75-watts of energy when it is turned on. Wattage is the actual power consumed, or “true power” that is recorded by the electrical meter and charged for by your utility company.

Volt-Amps (VA)

Volt-Amps are very similar to Watts, but are not the same thing.  A volt-amp is a calculated value, or assumed value that represents how much “theoretical power” should be present to be consumed.  Think of it this way: In the example above we said the Wattage of that lightbulb was 75-watts.  Well if we take the voltage (120v) of the circuit times the amperage (0.83amps) we get 75-volt-amps.  So in this light-bulb, the VA or volt-amps are equal to the wattage being consumed.  But that’s not always the case.

Say we have a 75kva transformer in a building.  75kva is equivalent to 75,000 volt-amps.  This could be because the transformer is a 480-volt transformer that can handle 156-amps of current flowing into it, or a 240-volt transformer that can handle 312-amps of current.  Either way by multiplying the voltage by the amperage we get the calculated VOLT-AMP rating of the available power present at the transformer. (480v x 156a = 75,000 va or 240v x 312a = 75,000 va)  We cannot simply say that this is a 75,000 watt transformer or 75KW transformer because the 75,000 figure represents a calculation for how much power is available.  This is called the “apparent power,” and must be represented in Volt-Amps (VA).  If we said this transformer was a 75,000 watt transformer then we would be saying this transformer will burn 75,000 watts of power when turned on, which is simply not true.  A transformer needs to provide a certain amount of power, that on the primary side is capable of producing a certain amount of power, and on the secondary is capable of producing the exact same power but with different voltage/amperage.

Remember the volt-amps are looking at the calculation of the circuit to see how much “apparent power” SHOULD be consumed, whereas the watts are the actual “true power” that is being consumed and charged to you by the power company.
 

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**Disclaimer – These videos are for training purposes alone, all work done on electrical systems should be done by a licensed and insured electrical contractor.  If you are not an electrician, do not attempt any of the work you are seeing in these videos.**

Comments

Erik Henry says

New too following you but really like what I’m wwtching and seeing!! Keep posting them!! Trying watch all of them all!! Thanks again

Dustin Stelzer says

Thanks for watching my friend!

Tim B says

OK, your explanation of Volt-Amps isn’t quite on target. In an AC circuit, as the voltage rises up and down in a sine wave (at 60 Hz in the US), but the current doesn’t necessarily follow. It may lead or lag the voltage, which is called the power factor. In your light bulb example, because a bulb is a purely resistive load, the voltage and current are aligned (in phase, or a power factor of 1.0), so Volt-Amps / Power Factor = Watts (approximately).
On the other hand, computers, LED lights, and electric motors have a load that is reactive, and this causes the current to lag (capacitive) or lead (inductive) the voltage. When the current leads or lags the voltage, the Power Factor is less than 1.0. The further the mismatch, the lower the Power Factor while the Watts stays the same. In a worst-case scenario, a device with a Power Factor of 0 would still have current flowing through it but consume no power (0 watts).

The reason this is important is a load with a low power factor draws more current than a load with a high power factor for the same amount of power transferred. Volt-Amps takes this into account so that proper size wiring, circuit breakers, and other means of power delivery are appropriately sized. Oh, by the way, the power company bills you for watts, not volt-amps.

Dustin Stelzer says

You’re absolutely right – I was trying to give a very basic understnading without getting into power factor or reactive power but I’ve gotten a lot of sparkies out there who didn’t like that I explained it this way, so I’m going to be remaking this video so it’s accurate. Thanks for speaking up, I’m never afraid to hear what I’m doing is wrong – means I’m learning. I mean it, thank you!

Richard says

Dude, once again thank u for making this subject so understandable. I am watching all your videos and they are the bomb. I didnt get into the electrical field until I was 55 years old. I dont have 20 years to learn what I need to know, your content is allowing me to get the knowledge at a much faster pace and I dont have to sit in a class room for hours at a time. Love what your doing, Please dont stop. Love you man.

Dustin Stelzer says

Thanks for watching my dude, I’m glad you get some value from what I’m doing. I’m trying to make it better as I go so it’s awesome to hear that people out there are enjoying it. I ain’t quitting any time soon my friend, I’m actually doing it full time so I can do it at the scale that I want to. Now I just need to get a team of people around me that can help me start pushing the envelope.

Stay tuned man, loads more coming!

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