To Off or Not to Off, That Is the Question

by Timothy S. Oey

Over the years, there has been much controversy about whether it is better to turn your computer off when it isn't used or leave it on continuously. This is not a simple question to answer, and as it turns out there are many different, but valid, answers. For the purposes of this discussion, we'll assume that "better" means that the computer will last longer.

The simple answer to this question is: It's usually best to turn the computer off whenever it won't be used for 8 hours or more. So if you use your computer frequently during the day, as many do at work, turn it on in the morning and off at night. If you use your computer less (a home computer for example), then it is even more advantageous to turn your machine off. Besides, in both cases you'll save energy.

Now if you'd like a more complete answer, or would like to know the details behind the above conclusion, please read on.

The following information was distilled from numerous conversations with engineers at Apple Computer, Conner Peripherals, and Quantum Corporation. The conclusions reached are not necessarily those of any of these companies or the engineers, but they are an attempt to derive information which the average computer user should find helpful.

To maximize the total number of successful operational hours for any computer device (i.e., maximize power-on hours), all computer devices (hard disks, CPUs, monitors, other electronics) should be turned on and left on forever until they fail. The number of power-on hours is what most engineers measure, and so they'll tell you to leave your computer on all the time to maximize them.

But this does not necessarily mean that you, the user, will maximize the amount of productive time you get out of the computer. For instance, during the night the computer may be on but it may not be accomplishing anything for the user. Let's call this productive time the user's perceived system life span. It's the span of time over which the user is getting useful work out of the computer. Users are probably more interested in maximizing their computer's perceived system life span than the actual number of power-on hours.

Regardless of the wear caused by turning your computer on and off, there are other factors that can have a much greater impact on your computer's life expectancy. It is very important to treat your computer with care. Although not the focus of this article, here is a brief list of common sense dos and don'ts that will help ensure a lasting and worthwhile relationship between you and your computer:

1. Keep it in a dust/dirt/smoke-free environment.

2. Don't spill things on it. Keep it dry.

3. Use a surge protector.

4. Don't drop it or jar it severely.

5. Use a screen saver to prevent monitor burn-in.

6. Keep it cool (room temperature) and out of the sun.

7. Don't block its ventilation slots.

8. Back up your files -- all systems are guaranteed to fail sooner or later.

Before we go on, be reminded that reliability is a complex probabilistic science. Yes, probability and statistics can be tricky, but they are necessary for determining when a computer is likely to fail. Manufacturers often use the term MTBF (Mean Time Between Failures) as an indication of reliability. This means that, given a large number of computers, the average one will run X hours before a failure occurs -- X being its MTBF. This does not mean that yours or mine will last X hours. It only means that ours will probably last about that long. There is no way that a manufacturer can determine exactly how long any given computer will last, just as it is not possible to determine exactly how long you will live, assuming normal circumstances.

Manufacturing defects tend to be the single largest cause of computer hardware failure. Manufacturers cannot guarantee that every device they make is perfect. However, some are better at minimizing the number of problems that occur. Apple(R) Computer, for instance, does an amazing amount of testing on all of its computers and peripherals, as well as keeping close track of failure rates in the field. Most well-known manufacturers do a good job in terms of design and manufacturing. You will nearly always be better off, in terms of reliability, if you buy equipment from reputable manufacturers. This cannot be stressed enough.

The whole point of buying a computer is to accomplish useful work. Be careful of the tradeoff between cost and the amount of useful work you will be able to get from your computer. The same goes for disk size, CPU speed, number of colors, etc. These don't necessarily mean that you will maximize the utility you get from your computer. Yes, there are worthwhile bargains out there, but you get what you pay for.

Many believe that turning a computer system on is the primary reason for failure, because most systems fail at this time. This is not necessarily true. It's just that this is the most likely time for weaknesses to become apparent, because this is when the computer system undergoes its greatest stress. Turning your computer on and off regularly (as prescribed above) may be even more advantageous than it first appears, especially in the first year of ownership. The stress of turning a machine on and off makes it more likely that any manufacturing defects will become apparent sooner rather than later -- and hopefully before the warranty runs out. Once manufacturing defects are weeded out, it is highly likely that a computer will run for quite a while before wear causes a failure.

Assuming the average benign environment, the most significant causes of wear, in rough order from most to least, are:

1. Heat

2. Power cycling (turning a machine on/off)

3. Power-on hours

4. Humidity/salt/airborne pollutants

5. Age (yes, some components incur wear even when not used)

Heat is a problem because electronic components may burn out if not cooled sufficiently. Excessive heat can damage any component, especially physically moving ones such as disk drives. Power supplies are sometimes a bit erratic for the first few microseconds when first turned on, resulting in initial power surges. Heating/cooling cycles can cause joint failures due to differing expansion properties between materials. The various wear factors have the greatest impact during power-on hours. Monitor phosphors and filaments eventually burn out. Humidity, salt, and pollutants can corrode various parts. Simple aging can also have an effect, although this is very small compared with the others. There are countless other effects related to the above causes -- to many to enumerate here.

Note that turning computer systems on and off vs. leaving them on may not really matter much for today's average user buying new equipment. Computer equipment is increasingly well engineered and reliable. It is much more likely to become obsolete than wear out.

Hard disk drives, for instance, are by far the most likely component of a computer system to wear out because they are mechanical and undergo fairly high stress. Most of the other components -- power supplies, monitors, logic boards, other electronics -- last significantly longer in comparison. Since hard disks are the weakest link in the computer, we'll focus on them for the remainder of our discussion.

Most high-quality hard disk drives are rated for an average of 20,000 on/off cycles and an MTBF of 25,000 hours or more. If you turn your machine on/off once a day, it will take 55 years to reach 20,000 cycles. An MTBF of 25,000 hours means that the average hard disk should last about that long, and 25,000 hours is equal to 2.9 years of non-stop running. Again, remember that these numbers reflect probabilities, not certainties. In testing, hard disks sometimes survive 100,000 on/off cycles and the equivalent of 1,000,000 hours of continuous operation. On the other hand, some fail much sooner.

<<1993 Update: Since the time this article was first written (Feb 1990), hard drive reliability has increased. MTBF ratings for most drives (as of 1993) are now in the 250,000 to 350,000 hour range. However, the basis for calculating these hours appears to have changed from an MTBF based on power-on hours to an MTBF based on total hours (on and off). The drive manufacturers may have incorporated typical use patterns into their calculations to boost the MTBF number and perhaps give people a number that on average is more meaningful -- 250,000 hours translates to 29 years of average use. Regardless, while hard disk drives may or may not be the weakest link in the computer anymore, and while MTBF ratings for other components may or may not have increased correspondingly, the logic above and below remains valid even though some numbers may have changed.>>

Now the question is: How much wear does turning a system on and off really cause? This is something that no one seems to have calculated yet. In fact, most will not even hazard a guess. However, let's take a crude and somewhat pessimistic guess that relates on/off wear to power-on-hours wear for comparative purposes. Let's say that the wear caused by turning a machine on and off is roughly equivalent to that caused by 8 power-on hours.

To do a rough calculation of how long a system will last if we turn it on and off each day, let's assume that a business computer is on for 8 hours each day, 5 days a week, 52 weeks a year, and that power-off time causes essentially no wear. Let's also assume that the MTBF for our computer is about 25,000 hours (this may be optimistic for a whole system, but it's good enough for our purposes). This yields the following equations, where <life span in weeks> is the user's perceived life span for the computer system:

<on/off wear in hours>

= <life span in weeks> * 5 cycles/week * 8 hours/cycle

= <life span in weeks> * 40 hours/week


<power-on wear in hours>

= <life span in weeks> * 5 cycles/week * 8 hours/cycle

= <life span in weeks> * 40 hours/week


<<on/off wear in hours> + <power-on wear in hours> = 25,000 hours


<life span in weeks> * 40 hours/week + <life span in weeks> * 40 hours/week = 25,000 hours

<life span in weeks> * 80 hours/week = 25,000 hours

<life span in weeks> = (25,000/80) weeks = 312.5 weeks = 6.0 years


If we leave the computer on continuously:


<on/off wear in hours>

= 0


<power-on wear in hours>

= <life span in weeks> * 24 hours/day * 7 days/week

= <life span in weeks> * 168 hours/week


<on/off wear in hours> + <power-on wear in hours> = 25,000 hours


0 + <life span in weeks> * 168 hours/week = 25,000 hours

<life span in weeks> = (25,000/168) weeks = 148.8 weeks = 2.9 years

Notice that the first case yields twice the life span of the second case, although the actual number of successful power-on hours is halved. A computer used less frequently would yield even better results.

Bottom line: Turn your system off when you won't be using it for 8 hours or more. But for the most part don't worry about it, because if you bought your computer system from a reliable manufacturer, it will probably last a very long time without any hardware failures. It is much more likely to become obsolete than it is to wear out. Remember that all systems will fail eventually, so keep backups.

Copyright (c) 1990, 1993 Timothy S. Oey. Tim Oey is a project manager at Apple Computer and the Forum Leader for BikeNet on America Online. He can be reached at, or Permission is granted to distribute this article electronically for free as long as it remains a complete whole. Please contact the author if you wish to re-publish the article in some other form.

Update: Tim Oey's email address can be found in Yahoo's email directory. It is not listed here so as to avoid spam bots.

Last updated 2000/11/24