I get asked this question every time I set up a new computer. Two schools of thought exist for the answer to this question.
Turning the computer off at night, when you are finished using it for the day, definitely saves on power. You can calculate that cost using the right tools, and the cost is all dependent on what you pay per Kilowatt hour. Needless to say, you can save some smackeroos on your electric bill by turning your computer off when not using it.
Leave the computer running 24/7. Some say this actually extends the life of your computer. When you turn the computer off, the electrical components on the motherboard cool down. Turning it on heats them up. Over time, the daily cool down, heat up cycle actually wears out the components of your computer and shortens the life of your computer.
With all of that said, I have three computers in my office that run 24/7. The desktop computer has been running that way now for 4 years. I occasionally reboot it to give it a refresh.
I have a Windows laptop, and a Mac laptop that also run 24/7. The Mac is 3 years old and the Windows is about 9 months old. Both run like champs.
Make sure you do these as well
If you choose to leave your computer running 24/7, make a point to dust it once in a while. The air vents will get covered in dust and they need to be cleaned out. Dust will make your computer overheat and generally run badly over time.
If you have a laptop, dust the bottom of it once in a while. Also, make sure it has air circulation underneath. Mine sit on laptop “arms” that have air holes in the bottom and they are elevated off the desk. Good air circulation can work wonders for a laptop.
I do, however, shut my computers down if I am going to be out of town for more than a few days.
Use a screen saver, or configure it to have the monitor dim and turn off after so many minutes. I use a screen saver that comes on after 20 minutes of inactive time, and the screens dim after 2 hours of inactive time.
So now you should be able to make an “informed” choice on whether or not to leave your computer running when you are not using it.