Basic AC wattage question: am I doing my math right?

That is right as far as it goes, and assuming that your meter reads correctly in RMS it will give you an upper bound.

The "real" number depends on the power factor of the device, which is probably less than 1. AC power can be "reactive", where the current is out of phase with the voltage, and the energy, instead of flowing into your device, is just swishing in and out at 120Hz (or 100, depending on where you live).

Unless the answer is critical, though, your number is probably more than good enough.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Hey, Tim, thank you for your response. It's not critical, I just wanted to know. The device is a television that is plugged in but not turned on. Somewhere I once read that the power consumption of televisions that are plugged in but not turned on is still significant (as far as the whole United States is concerned). I have always wanted to verify that by testing my own TV and finally have gotten around to testing it. Then I thought it would be fun to check how much it costs me in terms of money, hence what I've done here. FWIW, the Hz is 60.

If the number I've come up with is 0.0496 kWh per day and if working the number in a more precise way as you have alluded to (considering power factor and whether or not the device is subject to the reactivity of AC) would only get me maybe 10% more precision then my number is fine for my purposes. I only need it to be close. If my method gets me 0.0496 and the best way gets 0.0495 then I don't need to do it the best way. If my way gets 0.0496 kWh/day and the best way gets me a number like 2.6 kWh/day then I would want to do it the "best" way because the inaccuracy is so great that it voids the purpose of me doing the testing at all.

Thanks again for your reply.

--HC

"HC"

** Everything depends on what that "device" is - you fool.

Pathetic of you to hide what it is.

....... Phil

"HC"

** Don't thank someone for supplying bad advice. ** Then your test method is completely worthless.

** So you prefer vague and false information. ** The likely error is hundreds of percent.

Not using a true rms meter means your amp reading may well be out by factor of 2 or 3.

The power factor of the load is also unknown and can alter the result by similar factor.

A special wattmeter is needed measure the standby energy consumption of such an appliance.

See the one in the pic here:

..... Phil

Wow, so you're this group's neighborhood troll. Nice to meet you. Hey, when you grow up your dicky might get bigger than it is now. Be patient. In the meantime, find another way to compensate for it.

--HC

Hey Phil.... Merry Christmas and Happy New Year.

Now, do your best insult. Have at it Big Guy.

Ignore what he says at your peril.

"HC" = Google Groping moron.

** Wow, so you're ANOTHER of this group's FUCKWIT trolls.

Peeeeeuuukkkeeeee ....

..... Phil

Don, I appreciate your comment and I will look for more information to try to get a better understanding of my question and the answers I've received, including Phil's. I just didn't appreciate being spoken to harshly for asking a simple question and being maligned for leaving out details that, in my ignorance, are perhaps important.

--HC

Does mommy know you're using her computer? Parental filters are so underutilized.

--HC

"HC" = Google Groping TROLL

** You are one, pig arrogant, posturing bloody idiot.

PISS OFF

....... Phil

"HC" = another Google Groping SHITHEAD

** Wot a pathetic wanker.

Yaaaaawnnnnnnnnnnnnnnnnnnnn ...

....... Phil

Howdy, Phil. Did ya get some more of those majuhkul stereo resistors for Christmas? You know, the ones you love to use to parallel all kinds of stuff?

I bet you asked Santa for a few rolls of that thar oxygen-free hookup wire, too.

Didja get that, too?

Be careful now, you don't want to cause a fire at the home.

Merry Christmas, YOF.

"Joke"

** This congenital dope is drowning in the shallow end of the gene pool.

...... Phil

Phil has a problem; always has, always will.

Your math is right for a simple (ie, resistive) load, but might be off by, say, 2:1 in either direction, depending on the actual load waveform and/or power factor.

John

Oh goodness, playing near operating farm equipment, again?

And got yor'n britches caught by a moving tractor?

Hey, John, the more I've been reading the more I think my math and measurements have been badly wrong because they're rooted in DC methodology which seems to be quite different from the AC world (or can be for equipment/circuits that are not 100% resistive). So, as you say, I could be quite wrong by just simply not knowing the intricacies of AC power flow. I found a formula for calculating AC current consumption on single-phase which is P = V x I x cosine Theta which is great except that Theta is the "power factor angle" of the equipment which I don't currently know. I approached this problem thinking like I do about DC and I've found that it is wrong and the problem is considerably more complex. I'm going to do a lot more reading about this and "reboot" my whole test.

Thank you for your reply.

--HC

Check out the "Kill-A-Watt", it does *exactly* what you want. I bought mine on-line for about $20 (with free shipping!) for uses like this. I tested 2 different TV models of the same size (27") and similar age and found that one used nearly 16 watts when off, while the other used less than 8.

The Kill-A-Watt shows RMS volts, amps, watts, KWH, power factor, line frequency, and probably some others I've forgotten. You plug the Kill-A-Watt into the wall, then plug the thing you want to test into the Kill-A-Watt, and push a button to select what you want to read.

I've wanted a watt-meter for *years* and always figured I'd have to bite the bullet and build a crude one. But the Kill-A-Watt does more than anything I'd ever have built... and for only $20 !!! This is by far my favorite toy of the year.

(PS: I have no affiliation with Kill-A-Watt, just a delighted customer.)

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

No, even worse, a run in his panty hose.

John