Undervolting, and power consumption

How does Undervolting a CPU effect power consumption.

I am thinking about creating myself a small file server, for my lil' home network, and well, since it does not use much CPU power, I was thinking of Underclocking it, and also, undervolting it.

I was Going to use Either a Pentium D 805, or a Celeron D -- still have not made my mind.

But How would one calculate the Watt usage, if the CPU was undervolted, just so I get an idea how much less power it will use...

Unfortunately this site does not have modern chips in it's table but there must be other sites out there that let you insert the type of chip you have, then input you over(under)clock and over(under)volt and tell you the resultant wattage used.

^ The calculator you linked worked fine for me, just enter the values based on the formular.

Yes in my view undervolting does effect power consumption, in the fact when you take away a small amount of voltage, in return you save a significant amount of wattage usage. You may likely find that you could underclock just a little, that with a large undervoltage, and you could see a reduction in power usage upto 50%.
Here is another one with almost every modern processor ever made and the one your looking for.
http://www.extreme.outervision.com/psucalculator.jsp

That PSU Calculator is really Cool! I have the perfect PSU for my system!

find the max amperage of your processor and multiply it with the voltage to get max power draw.
I don't know where to find intel's max amperages tough, but AMD lists it on their site with the processor specs.

I wouldn't recomend undervolting a CPU, at least not by much. I work with DSPs and under volting them can make them very unstable and I would assume the same is true for a CPU since they are vertually the same thing. I have had it where a DSP refused to run correctly at 1.7 V when the core voltage was recommended to be 1.8 volts. I would just go with underclocking the processor, you will already be saving quit a bit of power by just doing that.

Well my Opty is Dual Prime95, Video Game, Music Encoding Stable at 1.1625V at default 2.0Ghz. Default voltage is 1.35V

I can drop from 1.55v @ 3.2GHz to 1.1v @ 2.4GHz super stable no problem at all.
That takes it from 92w stock down to 35w, thats one hell of a reduction (almost a third of the power usage), great if you just want to run some light background programs when your away from your system for long periods of time.