Does having PC 24/7 harm it?

Believe it or not, It is actually better for your computer to leave it running 24/7. Every time you turn it off and on the components on the board heat up and cool off. This heating and cooling causes expansion and contraction of the metals, which could in theory eventually cause a failure. However modern electronics are very durable and you are probably more likely to replace your computer before this becomes an issue. I would say to turn it off when you aren't using it so you are not wasting power. From a safety perspective, you are fine to leave it running as long as it has proper ventilation.

You mean, as opposed to loaning it out occasionally? I wouldn't do that. 

If You are not using It for Anything, then You should turn It off.   Its Not Good waste energy like that.

No........ and Yes

Yeah, heat kills the hardware but when the PC is idle the heat isn't excessive. The PC is in a lower powered state. You should be fine if the PC is free of dust and well ventilated. 

About all you have to worry about is the fans failing over time. Once a fan fails, then it needs to be replaced in order for the PC to work in it's correct state. If the fan has failed then the PC will overheat and either throttle or shut down. Replace bad fans and you will be fine. 

People who run with SFF PC's or they don't dust out their will have problems. 

It is actally better to leave your PC on but in sleep mode than to turn it off and then back on every day.

When I don't use my PC I let it into sleep mode. Does it helps if I need PC overnight if something takes long?

All electronics age through time, especially if used.  Heat buildup is the worst cause. If your "PC" of some sort runs all the time, it will not last as long as if it just sits powered off (power cord pulled out also).

There is a drawback--the "PC" has to be powered up to use.  The inrush of power is higher when the circuits are cold.  But not very much.

With no clues from you about what "PC", its age, its value to you to replace, what you might use it for (job, monitoring some process, just fooling around on Internet or with games), how often you might switch it on every day, there is no absolutely correct guess I can give about leaving it on.

As far as safety, any electrical device that overheats to a high enough temperature might start a fire.  This is particularly true of laptops that have lithium ion batteries.  Nickel-cadmium battery laptops are safer.

There's mixed perspectives on this...  There's generally 2 downsides with keeping your PC on 24/7...

1) Waste of electricity when idle & NOT in use...

2) Added wear on mostly mechanical (moving) components (like HDD & fan motors)

I will note that most computers will have power management settings enabled & will usually be on "Balanced" by default, which does the following:

* After 3 minutes / 15 minutes -- HDD's power down

* After 5 minutes / 15 minutes -- Monitor turns off

* After 15 minutes / 2 hours -- Sleep Mode engages

* After 3 hours -- Hibernation Mode engages (if available)

Please note that the low-end times are for laptops on batteries while the higher-end times are for laptops plugged in & desktops.  These settings can be tweaked to your preferences.

With these settings, power usage when idle decreases over time when your computer is not in use & will eventually drops to virtually zero when Hibernation Mode engages (as it dumps the contents of your RAM to your storage drive, sets a special flag int he start-up sequence & powers down completely).  These settings also reduces wear on mechanical components as well (not as much as actually powering down completely, but comparable).

The UPSIDE with keeping your PC on 24/7 is that it allows all the background services run, which includes download & installing updates for your programs & OS (which likes to do such stuff when you're NOT using your PC) & performance maintenance like defragging your HDD's, backing up files to an external storage drive & optimizing your Optane cache (if your computer has such a module).  This is stuff that most prefer NOT to happen when you're using your computer.

Such activities are usually scheduled & will trigger an interrupt on some inactivity timers & "waking up" your system from sleep mode.  HOWEVER, this generally won't wake up the monitor in the process.

One thing that some people do is to donate their idle processing power to a digital distributing cause (like Folding @ Home, which happens to have a COVID-19 project in the works... for one example), which keeps your computer active (preventing idle power measures to kick in) while you're idle.

Whatever you choose to do with your computer is eventually up to you, but I hope this sheds some light on the subject.

You smoke PCP 24/7? Yes that can and will harm you

I think that you missed something in that question.

Are you asking if it is okay to keep it on 24/7?

If that is the case, then no.  Most things are more apt to fail on startup, whether a computer, a car, or a light bulb.  Computer servers all over the world are on 24/7.They do use electricity though, so most people have them go into sleep mode when not in use.