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Kill A Watt: a question about how it reads wattage

post #1 of 9
Thread Starter 
This is in regards to the famous Kill A Watt P4400.

I have a portable space heater that has two settings: 1350W, and 1500W. So I tested it to cure my boredom about a week ago, and this is what I remember:
  • While on the heater's 1350W setting, the Kill A Watt showed roughly 400-something watts.
  • While on the heater's 1500W setting, the Kill A Watt showed roughly 900-something watts.
However, during the transition from 1350W to 1500W, it peaked at roughly 1350W (I think it was 1352) and then settled down to somewhere in the 900s. When I switched back to the 1350W setting, it didn't peak; it just dropped down to the 400s.

I have the Kill A Watt plugged directly into the wall outlet, and the heater was plugged directly into the Kill A Watt.


Now, I know there's nothing wrong with this heater because it replaced an old 1350/1500W heater which had the exact same heat output as this newer one. However, I can't test the old one because I got rid of it a couple of years ago.

To make me even more confused, my entire system's wattage readings are exactly like I would expect: my entire system peaked at a little over 500W during the most intense Furmark test (as intense as I could make it after running a few different Furmark configs). Now "my entire system" means that I have everything plugged into the Kill A Watt: speakers, monitor, cable modem, router, and a brand new telephone, all of which add roughly 62-63W to the power draw. So in other words, my computer alone was pulling roughly 440W at the wall during the final, most-intense Furmark test.

So, how can I explain my heater's readings?
Edited by TwoCables - 3/17/11 at 4:45am
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It's a computer!
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post #2 of 9
Is there any difference if you first allow for the heater to completely cool down and then turn it on with the 1500W setting?
post #3 of 9
when they say 1350 setting its its maximum wattage, but will not nessary need to use it continuously. It is only for pulse/spike draws. I'm guessing much like a psu where it wont get to 750watts continuously as it probably cant handle the draw, but for spikes yes.
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post #4 of 9
The likely explanation to me is the heater has some sort of thermal regulator, that something that monitors its temperature. It kicks on to 1300-1500W to get the unit to some specific temperature, then knocks down the draw to 400-900W, this keeps energy flowing through the coils without overheating the unit. If it is cool in the room the unit will hopefully be emitting heat well (thats the point of it) and cooling off, when it reaches some DeltaT from its setting it will kick up the wattage to 1300-1500 again until it reaches the pre-set cut off.

Likely if you start the unit from cold you will see starting watts much higher than 400-900 that then drop off when its hot. If you then measure the watts used while timing it, your average will be slightly higher than the 400-900 observed.
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post #5 of 9
Exactly as stated above, the rating is the peak draw, however, if it has some form of PFC (Power Factor Correction) that is known to somewhat confuse the basic power meters like the kill-a-watt. This also happens with computer power supplies when attached to the kill-a-watt. They read a bit low.
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post #6 of 9
Quote:
Originally Posted by allikat View Post
Exactly as stated above, the rating is the peak draw, however, if it has some form of PFC (Power Factor Correction) that is known to somewhat confuse the basic power meters like the kill-a-watt. This also happens with computer power supplies when attached to the kill-a-watt. They read a bit low.
Do you know of an easy to use cheap meter that will read computer power supplies correctly?

/interested
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post #7 of 9
The kill-a-watts I have found to be pretty accurate.

Anyway, I would hate to have one device trying to pull 1500 watts out of a circuit in my house. Regardless of what the heater says I doubt it pulls that much wattage for anything but the briefest of moments.
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post #8 of 9
Thread Starter 
Quote:
Originally Posted by TFL Replica View Post
Is there any difference if you first allow for the heater to completely cool down and then turn it on with the 1500W setting?
There is, and I was wrong about the 1350W setting being in the 400s; it's actually in the mid 500s. Here's a breakdown:
  • When starting cold and jumping straight to the 1500W setting, it peaks to roughly 1220-1230W and then settles down to the low 900s (like 910W or so)
  • When starting cold and using the 1350W setting, it just climbs to the high 500s and then settles down to the low to mid 500s.
  • When letting it sit at the 1350W setting for 60 seconds and then switching to the 1500W setting, it spikes to nearly 1350W and then settles down to the low 900s
  • When switching back down to the 1350W after leaving it at the 1500W for 60 seconds, it drops to the high 400s and then slowly climbs up and settles into the mid 500s.

I must be a geek because I'm having fun!

Quote:
Originally Posted by Voidsplit View Post
Do you know of an easy to use cheap meter that will read computer power supplies correctly?

/interested
I have the same question now because my PSU has Active PFC.
It's a computer!
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It's a computer!
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i5-2500K @ 4.5GHz (1.368-1.384V fixed voltage) ASUS P8P67 EVO B3 (UEFI ver. 1850) GTX 780 ASUS DirectCU II (1228 / 6300, 1.180V) G.SKILL Ripjaws X 8GB (2 x 4GB) 1866MHz, CL9 
Hard DriveHard DriveHard DriveOptical Drive
250 GB Samsung 840 EVO (C:\) 250 GB Samsung 840 EVO (D:\) 150 GB WD VelociRaptor Samsung SH-S243N 24x DVD Burner 
Optical DriveCoolingOSMonitor
Samsung SH-S203N 20X DVD Burner Thermaltake Frio Win 7 Home Premium x64 SP1 Retail AOC G2460PG (24" 1920 x 1080 144Hz G-SYNC) 
KeyboardPowerCaseMouse
Filco Majestouch 104-key Cherry MX Blues w/NKRO Corsair HX650 (Bronze, ordered on 12-12-2009) CM 690 Intellimouse Optical (1.1A) 1000Hz polling rate 
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post #9 of 9
Quote:
Originally Posted by Mygaffer View Post
The kill-a-watts I have found to be pretty accurate.
I concur. I have actually tested a few with a power quality analyzer meter, and they measure up pretty well.
    
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