This question gets asked a lot, and unfortunately there’s no easy answer. All sorts of factors must be considered, such as how much power your computer draws vs. how much your electric utility charges per kilowatt hour; how important it is to you that you have immediate access to your computer at all times; and on and on. Essentially, it boils down to your own personal preference. Here are some things to consider:
At a basic level, an average computer uses about 350W of power (including the monitor) or 0.35 kW. (This can vary depending on the size of your machine, what you use it for, etc.) Figure out how much your local utility charges per kilowatt hour, and then do the math to see how much electricity your machine would consume if left on all day. Rates fluctuate based on your city and the time of year.
Wear and Tear
Computer parts are designed to be extremely durable and last for years, but as with all machines that have moving parts, things do eventually break down. In a PC, the cooling fans (for the power supply and the CPU) are the primary components that will be in constant use if you never turn the machine off. The fans don’t have to work as hard if the computer is in sleep mode or is not actively running several programs while still powered up, but eventually they’re likely going to give out and will need to be repaired or replaced. Hard drives also continue to spin when the computer is not in use, and it can eventually peter out, too.
Too Many Processes
If you leave your computer on all the time, it is still running all manner of processes, even if no programs are open. Over time, these processes lead to memory errors that constantly build up, which can cause crashes, programs to freeze, and operating system lag. Shutting down your computer when it’s not in use can help limit that kind of poor performance, along with periodic sweeps with antivirus/malware software.
The bottom line? To save energy and give your computer a longer lifespan, shut it down when you’re not using it.