Re: Should your average pc be left switched on all the time?

The answer to this question is not entirely clear-cut because there are several competing factors that must be considered. For instance, it is widely recommended that you should not turn a computer off and then immediately turn it back on again (for example, to force a cold reboot). This advice originated in the days when microprocessors required a -12 volt bias voltage, which had to be turned on before the positive supply voltage. The power supplies in these computers were designed to apply the voltages in the correct order, but that only worked reliably if the power supply voltages were completely drained before it was turned back on. Thus was born the advice that if you turn off the computer, you should leave it off for at least 30 seconds before turning it back on. This was so that the capacitors in the power supply would have time to drain completely.

In modern microcomputers, the power supply doesn't provide a -12V bias, so it is no longer necessary to worry about that particular problem, BUT the complexity that used to exist in the power supply has been moved to the motherboard. Modern processors often use multiple power supply voltages (e.g. 3 V for I/O and 1.8 V for internal processor operation), and, depending on the manufacturer, there may be a requirement that these voltages be turned on in the correct order (I've never looked up the details). Similarly, other components on the motherboard may have particular requirements that are only met if the supply voltages appear as expected ("flash" BIOS chips are probably good candidates for this problem, so for some computers the advice you were given may be accurate). There is even the possibility that rapid cycling will cause the power supply itself to generate voltage surges that can damage components, although a well-designed supply shouldn't have this problem.

Thus, although rapid supply cycling is far less likely to damage a modern computer than it was in the "good old days", it still isn't a good idea. I recommend leaving the system off for at least ten seconds before turning back on (one system I tested does a warm boot if it has been turned off for less than ten seconds!).

Another aspect of your question is an old one that has been around much longer than microcomputers have: is it better to leave the system on all the time, or turn it off when I'm not using it? It is fairly difficult to come up with a firm answer to this one, in part because the answer can change every time the technology changes. In general, the most stressful times for electronic circuits are the surge of current that occurs when they are first turned on, and over-voltages ("voltage surges") that may occur because of lightning, large motors on the same electrical circuit as the computer, or even quirks in the behavior of the power supply when it is first turned on. This means you need to balance the stress caused by turning the system on with the stress encountered when there are nearby electrical storms (or other causes of voltage surges) while the system is on.

In practice, if you spend enough time with electronic devices, you notice that they usually fail when you first turn them on. A microcomputer seldom fails while you are using it: it is far more likely that you will turn it on some day and it will simply fail to boot properly. This is because the stress caused by the power-on current surge is much greater than the stress caused by leaving the system on for long periods. For this reason, the usual recommendation is to leave the computer on unless you won't be using it for a long time, or there is some specific source of voltage surges that you need to avoid exposing the system to.

The definition of "a long time" is where it becomes difficult to nail down a firm rule. In the days when computers were large and everyone had terminals on their desks, manufacturers recommended that a terminal that was going to be used all day should be left on overnight (with a blank screen). Since in my experience computers still usually fail when they are turned on, I still generally follow that advice: if I'm going to be using a computer regularly throughout the day, every day, I usually leave it on at night, BUT that is only if I think there is little risk of a nearby lightning strike causing damage, or a sudden power failure causing loss of files. What this means in practice is that if the system is on a good UPS then I leave it on at night. If not, I turn it off at night since there are a lot of electrical storms where I live. I generally leave my system on during the day if I expect to be using it again later in the day.

Some more thoughts that you might want to consider:

Hard drives: older hard drives were only designed to survive a year or two of continuous use, but modern drives are typically designed to have a minimum lifetime of at least five years of continuous reliable use, and are actually designed to have an average lifetime of over fifty years (i.e. Mean Time Between Failure = 500,000 hours)! Thus, you shouldn't need to worry much about leaving the drive running.

Power supplies: two common failures in power supplies are the switch and the fan. The switch, obviously, will last longer if you never use it. Fans bearings often become contaminated with dust or gummy oil. As long as they are left on, they work fine, but once they are turned off they get stuck and won't start again, so they often work longer if you never turn the system off. If your fan makes funny noises when you first turn the system on, and the sounds disappear after a few minutes, this is probably starting to happen.

Power surges: high voltage surges from external sources (lightning or large AC motors, for instance) are a major cause of system failure. Thus, if you are in a lightning-prone area, or have unstable AC power, you can reduce the risk of damage to your computer by turning it off and unplugging it when you aren't using it. You have to trade off the risk of damage from external sources with the increased stress caused by frequently turning your computer off and on. The best compromise, of course, is to get a good-quality UPS with good surge protection so you don't have to worry about lightning.

Brownouts: low AC voltage can damage your computer's power supply, so if you are in an area where "brownouts" are common, it is again wise to turn off your system when you aren't using it. A good UPS also solves this problem, allowing you to leave your system turned on during short brownouts.

Cold reboots: if you are in the habit of turning off a system then immediately turning it back on to force a "cold" reboot, you should understand that you probably won't get a true cold reboot unless you leave the system turned off long enough for the supply voltages to drain. On the one system which I have tested, I have found that it has to be turned off for at least ten seconds to guarantee that it will do a cold boot rather than a "warm" boot. Unless you really need a true cold boot, it is much better to use the reset button to restart a locked-up system.

I've seen at least one specification (for a hard drive) that suggests that one power-up is equivalent (in terms of aging) to ten hours of operation. This is, of course, only a very rough equivalence, but it illustrates why it is generally recommended that systems not be turned off and on frequently.

A UPS (Uninterruptable Power Supply) uses batteries to provide power even when there is a power failure, and if well designed also provides considerable protection from voltage surges caused by lightning or other sources, and from the under-voltage stress associated with brownouts.