Pentium 4 "C" @ 3.16Ghz
1024Mb PC-3200 DDR in Dual-Channel mode
160Gb 7200RPM Maxtor HDD with 8Mb buffer
MSI Neo-FIS2R i875p Motherboard
ATI Radeon 9700 Pro All-in-Wonder
BTCCD-RW 48X16X48 Generic 40X CD-ROM
Works 300W Power Supply
17" NEC Multisync FE-700 Monitor

Performing a Quake 3 Timedemo, the above system uses 144 Watts. Energy usage is what you will get billed for. Energy is expresses as watt-hours. To determine watt-hours, simply take the number of watts and multiply by the number of hours its used. How long is the computer on and running? If we assume it’s on for 8 hours in the 24 hour period of time, then the energy usage would be
144W * 8hrs = 1152 Watt-hours. This can also be expressed as 1.152 KWh. Change the numbers as needed for your specific situation.

The power supply rating is what the power supply can deliver on a continous basis. The rating doesn’t determine how much power it will use. The load on the power supply detemines the power consumption. So, although this computer has a rated supply of 300W in the example above, it is only using 144W.

And Im not going to sit here and do the math for every type of power supply available for a desktop… Too much work! LOL

also depends on if the computer goes "to sleep"

right click on the desktop, hit properties / screen saver /power
you can change if the hard drive + monitor goes to sleep or not

Mac computer doing nothing: 97 watts per hour

Mac computer with the monitor dimmed: 84 watts per hour

Mac computer with the monitor asleep: 62 watts per hour

Mac computer on sleep mode: 3.5 watts per hour

You can see from this one example alone how big a different such a small change can make. For Macs, changing your settings to sleep mode can save you approximately 93 watts per hour. Energy costs have been rising steadily in the last few years, so energy conservation is also cost efficient.

Computer monitors use 80% of the energy consumed by personal computers, so screen savers, which many people falsely believe save energy, are not an effective tool for energy conservation.

Laptops are significantly more energy efficient than desktops, using only 1/3 of a desktop’s energy per hour.

15-45 watts per hour compared to 65-150 watts per hour

Computers and monitors marked with Energy Star are designed to save power. However, they only save power when not in use. One of the problems with Energy Star is it gives people the false sense that they are not wasting energy by leaving a computer on all the time. The typical energy efficient computer, not counting the monitor, consumes from 15 to 40 watts in sleep mode. If you leave our computer on all the time, that means you are using an extra 16 hours a day and another 48 hours a weekend or 3328 hours a year. If we use an average of 25 watts, that means the computer will use 3328 x 25 = 83,200 watts per year just in sleep mode. The same computer uses about 150 watts an hour when operating.

For every watt of heat that each computer generates, it takes two watts of air conditioning to remove that heat. The typical CRT display uses 60 to 90 watts. We will use 60 watts for this exercise. At 8 hour a day and 5 days a week the computer is operational 1,300 hours. The combination of the computer and monitor is about 200 watts of power per hour or 260,000 watts. If we add the air conditioning, this could take as much as 780,000 watts a year per computer during operation and 83,200 watts when in sleep mode. That would be a grand total of 863,200 watts per year per computer. At .10 per killowatt, that is $86.30 cents per computer. An office with 100 computers would pay $8,630.00 for electricity.

NOTE: Running a screen saver means that the computer is not in sleep mode. The fact that the computer is displaying something means that the computer is in active mode and not sleep mode.

Energy efficient desktops and LCD displays can dramatically cut power consumption. An energy efficient desktop will consume about 20 to 30 watts of power. LCD monitors consume from 15 to 20 watts.

also see this…

http://science.howstuffworks.com/question658.htm

http://www.oasismontana.com/solar_home.html

Another major factor is whether you are using the energy saving features available for your computer? If so which ones? Most computers have energy saver settings in both the bios set up and under the screen saver settings?

In a worst case scenario a computer with a 300 W power supply under heavy use could use 300 watts per hour. That comes to 7,200 watt hours a day if my math is correct. With light usage a properly set up computer may average about 70 watts per hour or 1,680 watt hours in a 24 hour period.

Someone may have an easier answer to this question. However, the power supply on a computer is designed to adjust its output according to the demand of all the various electrical gadgets inside and connected to the computer. Therefore, the energy used varies depending on how the computer is used. The "average" desk top could use between 70 to 300 watts per hour.