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Multiply wattage by the number of hours your PC is on within a billing period. Multiply that by your cost per kW*h. That will give you a rough estimate of how much money it will cost you.
Be wary of the KAW figures. They're inaccurate when used with most modern PSUs with APFC.
Yes:Has anyone used a professional meter vs the KAW for a APFC PC load?
I understand that the values will be off by a few percent due to APFC, but +/- 5% is good enough for what most of us are using it for (ballpark figures and relative wattage estimates). For example, my system in sig idles at just 88 watts minimum, which seems in line with reviews.
Yes:
Here's Paul Johnson's post, PSU reviewer of HardOCP.com, about the inaccuracy of the Kill-A-Watt:
http://hardforum.com/showpost.php?p=1032190998&postcount=7
In addition, three other PSU experts backs up Paul Johnson's statement:
Oklahoma Wolf of JonnyGuru.com:
http://hardforum.com/showpost.php?p=1034843536&postcount=21
JonnyGuru of JonnyGuru.com himself as well as the senior PSU engineer over at BFG acknowledge the inaccuracies of a KAW (Post #7 in regards to Post #2):
http://www.jonnyguru.com/forums/showthread.php?t=5977
Redbeard of Corsair also acknowledges the inaccuracies:
http://hardforum.com/showpost.php?p=1032811067&postcount=22
So +/- 5% can't be expected.
I read through those but mostly they are concerned with the KAW overestimating efficiency when there's APFC. None of those statements show a big innaccuracy. As I understand it, Paul Johnson's post shows the KAW reading 215w when 219 true watts are being provided and 353 VA is the apparent power. This would put the KAW within 2% in that test, which is good enough for most of us.
The Brand Digital Power Meter shows the true figure of 353W being drawn from the wall on the AC side.
The SM-8800 is not measuring 219W. 219W is the load that the SM-8800 is putting on the PSU. Therefore, the PSU must be drawing more than that from the wall. However, the Kill-A-Watt is measuring a total draw of 215W, which is obviously incorrect since it is less than the load the PSU is under.So the SM-8800 (connected APFC power supply) measures 219 watts