The issue isn’t as simple as you might think; there’s no single answer to this perennial question.
My answer, naturally, is “It depends.”
There are two issues at play here: the power used by a computer left running 24 hours a day, and the stress on hardware components being repeatedly turned off and on.
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Power consumption is the easier question.
In most cases, standby is almost as good as turning your computer off completely. The computer uses a little power to keep itself ready for use, but components using the majority of the power, like spinning hard drives or display devices, are turned off.
Of course, turning the computer off completely prevents power from being consumed at all.1
Before you decide that standby or power-off is what you want to do to save electricity, it’s important to realize that your computer may already be taking steps on its own to minimize power usage when you’re not using it. Many individual components go into the equivalent of standby all on their own if they’re not used for some period of time. Hard drives may turn off after some period of inactivity. Screensavers eventually turn video displays off. Even a wireless mouse or keyboard may assume a low-power state when they havn’t been used for a while.
So, it’s quite possible that, depending on your computer, simply walking away reduces the amount of power being used.
Stress (the computer’s, not yours)
There is an argument that says leaving a component on continuously is less stressful than turning it on and off repeatedly. While you may save power while the device is off, the stress of heating up and cooling down when starting and stopping can contribute to earlier failure and required replacement.
The argument is that the cost (in terms of purchase price and the environmental impact of replacing a broken device) is so high that in the long run, it would have been more cost effective to simply leave it on all the time.
The counter argument is that most components are built with this in mind. As I mentioned, many components turn themselves off after some period of inactivity, whether you tell them to or not. The devices are built to power cycle fairly frequently.
And in almost all cases, devices are already lasting longer than we need them. Devices are most commonly replaced not because they fail, but because newer and more powerful devices are purchased.
What I do
As you can see, there’s no simple answer. There are arguments for both approaches.
My approach is to ignore all of the arguments above, as they more or less cancel each other out. Instead, I make the decision based on how I use my computers.
It’s no surprise that I am a heavy computer user, and for me, the answer is to leave almost all of my equipment running 24 hours a day. Not only does that keep everything ready and running for my use, it allows automated processes like updates and backups to happen in the wee hours of the night when I’m asleep.
What should you do?
My expectation is that an average user who uses their computer a few hours a day would most likely turn it off, or put it into standby, when it’s not going to be used for a few hours or more. That’s simply to minimize power use. It’s also not a big deal if you don’t.
If you use it longer, perhaps you leave it on all the time. Again, no big deal.
If there’s anything I might consider a tipping point in the decision, it’s backups. If leaving your computer on all the time allows you to automate the process and start taking backups where you otherwise would not, or would forget, then in my opinion, it’s totally worth it.
Backups trump everything.