Should I turn off my computer when I am not using it, or, is it better to leave it on all the time?

Here is a good  reason to turn off your PC when it is not being attended by someone.

This damage was caused by a lightning strike in the middle of the work day.

The strike caught the serial card (bottom card in picture above) on fire but the computer case fans kept running. The fire continued to get worse until the PC was unplugged.

The office personnel had to take the PC outside because it was still smoking 30 min after it was unplugged. If there had not been someone in the office, the building could have burned down.

If it does not need to be on, turn it off. Office computers are usually only used for 8 hrs out of the day and then left unattended for 16 hours while the user is off work. That's 16 hours of unnecessary electricity consumption, and wear and tear on the fans hard drives and CPU.

There is a rumor that has spread around the world that goes something like this: "It's all the heating and cooling of the electrical components that causes damage. That's why you should leave it on all the time". This is true in part. Turning on your PC when it's 32 degrees in the room is asking for trouble.

Temperature extremes can damage your computer, but if you use a little caution and common sense you can minimize this effect. Only turn on your PC if the ambient room temperature is between 55-90 degrees Fahrenheit. Outside of this range and you should let the room cool or warm as necessary then allow another 30 min to 1 hr for the internal parts of the computer to reach the same temperature as the room before you turn on your PC. Performance PC's and servers should not be left on when the ambient room temp is above 85 degrees Fahrenheit.