Leaving a computer on all the time will use significantly more energy than turning it on only when you need it, but if you have a reason to leave it on, like for instant access to your email or for downloading files, there are ways you can reduce the impact.

Normal Usage

If you actively use your computer for five hours a day and turn it off when not in use, at the average US electricity rate of 12 cents per kilowatt hour in November 2013, you would be spending about $2.30 a month for a desktop computer using an average 100 watts and a monitor using 30 watts.

On All the Time

By leaving the computer on all the time, with five hours of active use and without using the power-management settings of Windows, the monthly cost would increase to about $11.30.

Shutting off the Monitor

Powering down the monitor when you are not using the computer, but leaving the computer itself on, would reduce the cost to about $9.20 a month.

Using Power-Managment Settings

Allowing your computer to use Sleep mode and shutting off the monitor when the computer is not in use would further reduce the cost to about $3.20 a month, or just over a dollar more than shutting the computer down. The power-management options can be configured from the Control Panel in Windows. Setting the monitor to shut off after five minutes of inactivity and the computer to put unused systems into "Sleep" mode after 15 minutes of inactivity should maximize your energy savings.