Computer monitors waste over $900 million in electricity per year because:

60 % of all computers and monitors are left on at night!

40% of all monitors are not enabled for power management!

This electricity wasted is equivalent to 9 million tons of carbon dioxide emissions per year or the emissions of 1.5 million automobiles.

 So it is very important to know few tips to save electricity wasted by using PCs inefficiently

 A typical desktop computer uses about 65 to 250 watts.Add another 15-70 watts for an LCD monitor, or about 80 watts if you have an old-school 17" CRT.  Don't forget additional power consumed  by additional  related devices. 

 Most laptop computers use about 15-60 watts, far less than desktops.

Sleep & Screensavers :When your computer sleeps ("standby", "hibernate") the computer uses 0-6 watts. (So does the monitor.) You can set your computer to sleep automatically after a certain amount of idle time. Setting your computer to auto-sleep is the best and easiest way to save on computer energy use! Hibernate saves your workspace (all the open windows) and then turns your computer off, so it saves more energy than standby, but a hibernating computer takes longer to wake up. A screensaver that shows any image on the screen doesn't save any energy at all == you save energy only if the monitor goes dark by going to sleep.

It also doesn't take more energy to start a computer than to keep it running. The only extra energy it takes to start a computer is the two minutes or so it takes to start up, which is barely different than any other two minutes' of use. You'll always save energy by turning your computer off when you're not using it. Of course you don't have to turn it off since you can easily use the sleep or standby mode instead.

The myth of "turning it off uses more energy than keeping it on all the time" exists for just about every device that exists, and it's wrong in every single case, in practical terms. (Meaning, you will never, ever, ever see any savings on your electrical bill by keeping something on all the time vs. turning it off. .) You will always save electricity by turning your device off when you're not using it (or sleeping it, if it's a computer).

Few advisable tips:

• Set the Power settings on your computer to automatically go into Sleep/Standby mode after 15 minutes or so of inactivity. If you do nothing else, do this.
• Use a laptop computer. They use lots less energy than desktops.
• If you use a desktop, use an LCD monitor. They use lots less energy than CRT's.
• Turn your computer off when you're done for the day.
• Use a power strip so you can easily turn off all your computer accessories at once.

Source : http://michaelbluejay.com/electricity/computers.html