One of the questions that we are commonly asked is whether computers should be switched off when not in use or whether they should be on permanently. It’s a fair question, particularly when you see that so many professional organisations leave their machines running 24 hours a day – they never turn them off.

However, just because these large organisations do it does­n’t make it the right thing to do. Unless computers need to be accessed 24 hours a day, they should not be on permanently. Why? First, and this is a topic that we have touched upon in the past, if a fault occurs in a computer or more likely, in the video monitor, and if no-one is present to turn it off, it could cause a fire. It does happen! In fact, this month one of my own TV sets had a component failure which produced a lot of smoke and un­doubtedly it would have caused a fire if my wife had not been in the room to switch it off.

I have seen banks of machines left on in rooms where there are sprinkler systems – so if one machine catches fire, a whole lot of them get a bath!

Second, if machines are left on after hours and a thunder­storm occurs, they are vulnerable. And you can have all the electrical protec­tion that money can buy and it won’t mean a thing if the electri­cal supply outside the building gets a direct hit. There is only one sure way to protect a computer against lightning strikes and that is to disconnect it from the mains supply.

Third, computers use a lot of electricity if they are left on permanently. In our office at SILICON CHIP there are 11 com­puters and they are only switched on during the day. Only the server is left on permanently, to do after-hours backup. And if a thunderstorm is likely to hit overnight, we switch the server off too.

Just think about the power consumption of 11 computers left on permanently. A typical computer with its monitor will consume around 200 watts or more. Leaving 11 of them on perma­nently would be equivalent to running a 2.2kW radiator all the time. Now if we left such a radiator on in the office all the year round, you would say we were insane! You’d be right.

The cost of running all our computers for 24 hours a day instead of about 10 hours a day, at 10 cents a kilowatt-hour, is over $1100 for one year. That is not allowing for the extra power we would use for air-conditioning and yet some organisations run hundreds of computers all the time. Sure the dollar cost may not be huge relative to their overall expenses but it probably also reflects their generally slack approach to cost control.

One of the arguments commonly raised in favour of running computers continuously is that because they run at the same temperature all the time (not being cycled on and off), they are more reliable. Nonsense. A CRT in a video monitor has a typical brightness "half-life" of about 10,000 to 15,000 hours. 10,000 hours is not much more than a year. In effect, leaving a monitor on permanently (with or without screen-saver) reduces its life by a factor of about three. Hard disk drives also have a finite life – why shorten the time to their ultimate failure?

Even if the cost of replacing a failed computer is regarded as small, the time and cost of getting a computer back in use with all its software loaded can be very considerable. Most organisations do not take that into account when they take out insurance.

No, whether you have one computer or a hundred, it does not make sense to run them all the time unless they are being used all the time. Switch them off and pull the plug out of the wall socket. And if you must run computers all the time, at least switch off the monitors.