Mike,
It is really tough to say. Most computers use around 300 watts of power running full tilt. Meaning the monitor is on and the hard drives are spinning. With the advancement of power saving hardware, firmware, and software that number is probably high. I have seen computers in "sleep mode" that use only about 25 watts and I have seen others that use 400 watts. It all depends on the size and configuration of your computer. Anyway you look at it, you are only paying ConEd pennies a day to let your computer on. For example, a 300 watt usage running for 24 hours at $.07 per kilowatt hour will cost you about $.51 per day or about $15.00 per month. However, I will say that leaving a computer on continuously can be very hard on the harddrives and other components. Kinda of like letting your car run continuously. Hope this helps.
PS - Here is a good appliance calculator Joe posted in the NEC forum. It may be useful to you. It does have a listing for computers.
http://www.powerhousetv.com/calculator/index.php3 [This message has been edited by bigbluewirenut (edited 01-14-2002).]
