"How much electricity does a PC use? My daughter never turns off her PC and monitor, and her student roommates are concerned that she's adding too much to the electric bill. I suspect that their hair dryers use more power than the PC. Is there a way to calculate this usage and its cost?"
Sure, minute for minute, a computer and a monitor use a lot less electricity than a hair dryer, but I sincerely doubt that these students keep their hair dryers humming "24x7"!
How much power a computer consumes depends on its processor, components, and design and the amount of time it is actively in use. Not uncommonly, a computer designed to consume up to 200 watts (exclusive of the monitor) draws about 100 watts when in use and 50 when idle.
The real power hog is the monitor. A 17in monitor that's left on all the time can easily use 160 watts; even a 14in model can consume 100. Do the sums: At an average cost of 12 cents per kilowatt hour, a 17in monitor that never goes to sleep will cost over $150 a year to operate.
If your daughter must keep her PC on, she should use its power-saving features. Unless her monitor is more than three or four years old, it is almost certainly Energy Star compliant. She can set the power saving options by right-clicking the desktop, selecting Properties, and clicking the Screen Saver tab. She'll get options at the bottom of the window to send the monitor to standby mode - where it uses about 20 percent of full power - or to shut it off almost entirely during idle usage.
Your daughter is less likely to have a computer that goes into power-saving mode. Tell her to run the following sequence: from the Windows desktop select Start-Help, click the Index tab, and type power. Double-click Suspend command, showing or hiding. In the resulting Help window, click the arrow icon. She'll get either a pop-up box saying that her computer doesn't support power management or a dialogue box of options. But today's computers aren't all that energy efficient; most of them still gobble up about 30 watts even when powered down.