Watts vs VA

Question

Why is power draw measured in WATTS ( which is volts multiplied by amperage ) but power production or power sources are measured in VA ( volt amps ). Are they not identical? What’s the reason for the difference.

In: Engineering

Answer 1

I will try my first ELI5 answer.

Take your little red wagon, some string, and your little brother and go to the bottom of a small hill. Sit your brother on the wagon and tie one end of the string the string to the handle.

Now, grab the string a short distance from the wagon and slowly walk up the hill, pulling the wagon behind you. Be careful not to jerk or pull too hard.

The strength the string needs to be, so it does not break while you are steadily dragging the wagon, is the Watts.

This is slightly like a resistive load like you’d find with a light bulb or a heating element (as others have mentioned)

Now go back to the bottom of the hill. Position the wagon and your little brother in the same place but this time walk back up the hill with the other end of the string.

Pull the wagon up the hill hand over hand.

Each pull brings your brother a bit closer and the wagon may even coast a little bit before your next pull.

This is like an inductive load like in an electric motor or the alternating current in an AC circuit (which ‘tugs’ 60 times a second).

Unlike when you slowly pulled the wagon up with you, in this case you are tugging repeatedly on the string over and over to get the wagon up the hill. This means the string sometimes needs to be a little bit stronger in order to not break when you pull each time. This is like the Volt-Amps.

The amount of effort it takes to get you, your wagon, and your brother to the top of the hill is about the same in both cases.

This is why, in electrical systems, wiring and equipment that needs to handle AC current or feed power to motors, etc, are rated in VA because at certain points in time, they need to handle more. VA is always larger than W in these cases. Situations where there is a steady load VA = W as there is no ‘tugging’.

Edit.. Apologies, I kinda broke rule 4 I think.