Compaq Presario 6450NX Desktop PC?http://h10025.www1.hp.com/ewfrf/wc/document?docname=bpb11683&tmp_task=prodinfoCategory&cc=us&dlc=en&lc=en&product=240121
Power Supply - 200 Watts
That is the supposed Maximum Wattage the computer will use per hour.
Variables come in to play, however;
1) Back when the computer was made, the Power Supply manufacturers were 'fudging' the real maximum rated Wattage.
True ratings were more like 60 to 70 percent, of what was actually stated.
Divide 200 by ten; take it times;
6 for 60 percent. 120 Watts
7 for 70 percent. 140 Watts
So, the Actual Maximum Rate Wattage, could be as little as 120 Watts, or a little more than a 100 Watt light bulb.
2) Second variable is what is the computer being used for?
Surfing the internet? About 100 Watts.
Playing an intensive game, (Yeah right. Not on that computer), or using an intensive program?
Then, you may be using the Maximum rated Wattage,
120 to 140 Watts.
So,........the hourly wattage rating, could be as low as 100 Watts per hour, or perhaps up to 120 to 140 watts per hour.
Also include the power used by the monitor.
If you do not have the specifications of the monitor, see if you can look them up online.
My HP 2009M uses 47 Watts, (2 Watts when in Sleep Mode),http://www.newegg.com/Product/Product.aspx?Item=N82E16824176112
My other monitor I use (Dual monitors), is an RCA 26 inch HDTV,
Uses 82 Watts,http://reviews.cnet.com/flat-panel-tvs/rca-l26hd31/4507-6482_7-33346932.html
(And the power supply for my computer is 500 Watts.
Hmmm,............no wonder the little wheel spins so fast in my electric meter! lol!)
Add the two together to get a basic conclusion, of what Wattage per Hour your computer system is using.
Don't forget to include printer/scanner/fax machine/copier, WHEN being used.
For additional questions please post in a Comment.