# watt vs. amp vs. volt

Discussion in 'General Electronics Chat' started by samjesse, May 12, 2011.

1. ### samjesse Thread Starter Senior Member

Sep 14, 2008
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0
Hi

In A/C circuit where volt. and amps are 90° out of phase. and since w = volt. x amp, would it be correct to say that the watt also takes a sin wave with twice the frequency of the volt or amp?

If so, how would the elec. company calculate how much watt. I used?

thx

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,165
1,121
Whether power draw is a sine function or not depends on how the power is being used.

In a purely inductive or capacitive load, the waveform would be of a sine form. In reality the equipment usually only draws power at the peaks of the sine wave and therefore the wattage waveform would be distorted from a sine wave shape.

If you think the power company can't meter that type of usage, just give it a try and see. The meter uses various techniques that do not rely on phase difference between power and voltage. The most common being flux coupled eddy current generation in an aluminum disc. Power meters for large industry are different and can tell the 'power factor' of the companies equipment, which is charged at different rates than residential power.

3. ### russ_hensel Distinguished Member

Jan 11, 2009
821
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Also try not to confuse the units ( like watts ) with the concepts they measure ( like power )

Dec 26, 2010
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If the voltage and current were exactly 90° out of phase (which would require a perfect capacitive or inductive load), the average power would be zero. You would not be consuming any power, only reactive volt-amperes.

5. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Yea, you'd be surprised what they can read off our meters. (we have two as we've got two 6,000 A 480V step-downs outside on pads)

Not only can they tell the power factor but the highest peak demand at any one point of time during the month. As I watch the digital displays roll though their readings there's stuff in there I don't even know what is.

I'm also on "time of use" and we're currently converting about 250 ea. 400W Metal Halide fixtures to 105W CFLs that are designed to operate off 277V - makes if far easer that rewiring the entire building or hanging 4 ea. x high output T-8 fixtures in their place.