Will shutting down my PC extend the life of my power supply?

Not really, as far as I am aware. It will save power (if you haven’t set your computer’s power settings up properly, like most people, though, it might not), but the biggest source of
stress on electronic components that I know of is turning them on and off again thousands of times.

@the100thmonkey Is correct that power-cycling does put some stress on the power supply. But so does leaving it on. And manufacturers have made huge progress in the past few decades in accommodating and compensating for temperature stresses in the components, connections and circuit boards. Unless you got the cheapest model available, your power supply and computer circuits are made to take it. Also if there is any chance of lightning strike in your neighborhood your machine is less vulnerable to damage if it is off (not completely protected unless you unplug it but safer) — you should have it on a surge protector at least anyway.

If you actively use your computer twelve hours a day then, heck, leave it on 24×7. But if you are using it a more typical few hours a day then I’ll recommend turning it off when you’re done. I turn each of mine on when I need it and off when I’m done with it.

@dabbler Mostly correct. That begs the question of what sort of computer it is, which determines whether it has decent, new components or older/cheaper ones. I know that my power supply can handle far more than the rest of my computer; it’s a 610W Cooler Master in an otherwise stock, low-end 2009 Gateway. I doubt that the sort of advances in countering thermal stress made it that get down the food chain three years ago, so I treat it a little gentler than I would a fully-custom rig, though not as delicately as I did my 2004 Northwood which was made with even less advanced manufacturing methods.

As far as I can tell from the trade rags high reliability makes it pretty far down the food chain, if only because it’s cost effective – it costs too much to honor warranty if the machines are crap.
The only recent major brand I’ve seen that was downright shoddy was the first few years of eMachines, the insides of which look like the junk you’d find in a furby. I think they even bumped up their game a bit from their first offerings but not that far.
These days if you bought the cheapest machine at Walmart then you’d probably be stuck with junk, but any known major brand [Dell, Gateway, ASUS, Acer, Lenovo, HP (not counting the cheapest HP, they are junk) ...] I think you could power-cycle those thousands of times before that caused a problem.

Basically I wouldn’t worry about power-cycling a typical consumer-grade computer twice daily (morning session / evening session). Your mileage can vary.

To be fair, my Gateway has a fanless GT240 graphics card with a big-ass passive heatsink dumping a lot of heat into my case, so I’m a little more paranoid than I probably need to be. I haven’t had any issues in the three years I’ve had it, which makes it more robust than many previous systems I’ve owned. Considering that I stress my systems more than most people (I’m a gamer), you’re probably right, @dabbler. I’m just a cautious type that draws a sharp line between “probably” and “definitely” because I’ve seen some pretty spectacular “one in a million” system failures.

Of corse it will extend the life. The MTBF (Mean time between failure) is based on POH (Power on hours).
The power supply is certainly in the top 3 of failure parts that make up a PC.

@blueiiznh And what is the MTBF of the average user? I’ve seen some computer users fail quite frequently…

@jerv It really doesn’t matter. The question is whether a given part will do better constantly powered on or power cycled once a day.

@jerv “MTBF of the average user” FTW ! @blueiiznh woosh !