(In)Accuracy of a Kill-A-Watt for measuring low power usage

[michaelb]

Racer X wrote:
I have always run the Kill a Watt setup for a day or two, and divide the power consumption by the time to get average consumption.

You could easily check a freezer and read it while the compressor is running, or not.

I am also confused by the math and how this works. I don't pay attention to the instantaneous demand. I plug it in and let run for a period of time, days or weeks, and then divide by the hours. Is this saying that method is not accurate?

[freeradical]

AllGold wrote:
I could be wrong, but I think the VA (Volt-Amp) setting gives you a little more "resolution" than the Watts setting.

Not exactly. Watts refers to real power, while volt-amps refers to apparent power. In a DC circuit, you simply multiply voltage times current to get watts since the voltage is constant. In an AC circuit, the voltage is constantly changing, so you multiply the instantaneous AC voltage times the instantaneous current, and then integrate those results over a period of time.

In practical terms, both numbers matter, but the volt-amperes specification is more important when it comes to safety, and the watts specification is more important when it comes to figuring out your electric bill. This is because you buy power from your electric company in watts (Kw/h actually). The volt-ampere specification tells you what gauge of wiring you need to safely run some equipment. Since we get AC power at 120 volts rms, you simply divide this into the volt-ampere specification to get current. The maximum current is what you need to safely making a choice of wire gauge.

[Racer X]

michaelb wrote:
[quote=Racer X]
I have always run the Kill a Watt setup for a day or two, and divide the power consumption by the time to get average consumption.

You could easily check a freezer and read it while the compressor is running, or not.

I am also confused by the math and how this works. I don't pay attention to the instantaneous demand. I plug it in and let run for a period of time, days or weeks, and then divide by the hours. Is this saying that method is not accurate?
you are doing it right. Same method, different words.

[Bill in NC]

why worry?

switching power supplies use so little power with no load anyway.

[OWC Jamie]

Bill in NC wrote:
why worry?

switching power supplies use so little power with no load anyway.

but some use a lot less 'little' than others and the cume of dozens of these all over the house adds up.
It's nice to prove they use or don't use as much as you believe they do.

[Bill in NC]

1/100 of a watt?

again, why worry?

[Winston]

Bill in NC wrote:
why worry?

switching power supplies use so little power with no load anyway.

A lot of ours are not switching power supplies. I'd say most are not. I've not done a count, but we have a bunch around the house and I'm working to get a handle on what exactly it means to have them on all the time. Could easily be a couple of hundred watts standby.


- W

[AllGold]

freeradical wrote:
[quote=AllGold]
I could be wrong, but I think the VA (Volt-Amp) setting gives you a little more "resolution" than the Watts setting.

Not exactly. Watts refers to real power, while volt-amps refers to apparent power. In a DC circuit, you simply multiply voltage times current to get watts since the voltage is constant. In an AC circuit, the voltage is constantly changing, so you multiply the instantaneous AC voltage times the instantaneous current, and then integrate those results over a period of time.

In practical terms, both numbers matter, but the volt-amperes specification is more important when it comes to safety, and the watts specification is more important when it comes to figuring out your electric bill. This is because you buy power from your electric company in watts (Kw/h actually). The volt-ampere specification tells you what gauge of wiring you need to safely run some equipment. Since we get AC power at 120 volts rms, you simply divide this into the volt-ampere specification to get current. The maximum current is what you need to safely making a choice of wire gauge.
The reason I qualified it so much is because I wasn't going on actual knowledge of how that stuff works. I was basing it off of observation. I noticed on some low current devices that I was testing, 3 VA equaled about 1 Watt. That doesn't work with higher current devices so maybe it was all about the inaccuracy of the Kill A Watt under low current.