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Wattage vs. VA (Volt-Amps) and what's the difference...



Power in a DC circuit is expressed by the formula (P=IE).  Where (P) is the power in Watts, (I) is the current in Amps, and (E) is the voltage in Volts.  We all feel safe and comfortable with this power formula.  So what’s this (VA) designation we keep seeing on transformers and AC power supplies?  The key factor in the last sentence is “AC”.  For most applications of AC Watts can still be used as long as there isn’t a lot of the AC “stuff” going on known as reactance. We all know that when AC gets involved, things in electronics get more complicated. (VA) is the symbol for Apparent Power. Apparent Power takes into consideration all that complicated AC “stuff” I just mentioned.


The power in an AC circuit dissipated by a purely resistive load is called “true power” and is expressed by a formula that looks a little like the one we just mentioned for DC.  For AC the “true power” is given by the formula (P = Erms Irms).  The “rms” simply means that with AC we have to use the average values of the variables for a sine wave.  OK, so far so good.  Now, what about the AC “stuff”.  Reactance occurs when there is a significant amount of Inductance or Capacitance in a circuit. Both these components cause the voltage and the current to become out of phase with one another.  Yes reactance does draw a strange sort of power that doesn’t create any heat and doesn’t do any work, but it is power nonetheless and can zap a little transformer if it gets too high. The formula for this reactive power is Preac  = Erms Irms sin q.  q is the angle between current and voltage, given with reference to the voltage.


OK, so how do we come up with the Apparent Power (VA), the total power we can draw out of our little transformer without smoking it?  If Pa represents the apparent power in an AC circuit, Pt represents the true power, and Pr represents the reactive power, then (Pa2 = Pt2 + Pr2). Or:


Pa = ÖPt2 + Pr2


Unless you try to run your air conditioner off a wall plug transformer (large inductive component), or you want to send a lightning bolt across the room (large capacitive component), for most purposes, and I will let you define most, VA and Watts can be thought of as the same. You see, the transformer manufactures just didn’t want to get sued if you try to start your car with a small 12 V DC transformer.  Of course you wouldn’t try that in the first place but I am sure someone would.

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