When did your Mac last start up? Mine’s only been running continuously now for nearly ten days, but unless something forces me to, I don’t intend restarting it for another couple of weeks, and only shut it down completely a couple of times a year. Which is better, then: a daily boot, or leaving your Mac on as long as possible?

Traditional arguments about this have been based on the last generation of computers, with internal hard drives. I’ve been leaving my desktop Mac running constantly for many years now, and started doing so largely to reduce the risk of hard drive failure. This iMac Pro no longer has internal storage which spins platters, so it’s time to reassess what I do.

Factors to consider include:

• Which model it is. Laptops are engineered for intermittent use, with periods of sleep or shutdown in between. Even if you’re using a MacBook Pro as your desktop system much of the time, you’re very unlikely to be interested in running it continuously.
• Whether any part of your system still uses rotating disks. Although my iMac doesn’t contain them, it backs up to an external RAID which contains four conventional hard drives.
• Pattern of use. If your Mac is unused from Friday evening to Monday morning, it’s likely to make sense to at least shut it down for that period each week.
• Location and your presence. Similar considerations apply when the Mac is in an office which is only occupied during working hours.
• Servers and services. If your Mac provides services to other Macs or devices, it needs to be running when those services may be required.
• How often you replace it. I aim to replace my desktop when its AppleCare runs out, but many users will want a longer life.
• Energy consumption and cost. Even when sleeping, a desktop system uses electricity of course.
• Hardware reliability.
• Reliability of services (power, heat, cooling) and provision of an uninterruptible power supply (UPS). Desktop Macs should always be protected by a UPS, but when they’re unattended for long periods you may want one with a higher specification to cope with mains outages when you’re not there. Automatic shutdown is essential, and needs to be tested too.
• Security. If your Mac is going to be unattended for long periods, you need confidence in its physical (local) and Internet security.
• Ease of access. If you do have access out of ‘working hours’ but shut your Mac down then, you are unlikely to want to start it up just to check something out quickly over an evening or weekend.

The biggest argument in favour of leaving computers running constantly in the past has been to minimise the risk of mechanical failure of hard disks. Evidence shows that repeatedly spinning up and spinning down rotating hard disks reduces their working life, on average. Leaving a disk running at constant speed at all times generally results in longer life before mechanical failure. That should be a key consideration if any part of your system relies on conventional hard disks.

One way around this is to use networked storage for backups; you can leave that storage running constantly, but still shut your computer system(s) down when you wish.

It’s generally accepted that electronics which warm up in use, as computers do, tends to run with fewer problems when they are left running at normal working temperatures for long periods. One factor which has been implicated in this is the use of lead-free solder. It has been claimed that repeatedly cycling the temperature of some components soldered without the use of lead – which has been banned throughout the EU for more than a decade – increases the risk of joint failure.

However, since lead-free solder and its use with surface-mount components have undergone considerable improvement over that period, it’s hard to know how true or significant this might be. Most electronic engineering advice, though, continues to recommend that systems are left running in stable conditions as much as possible to ensure their maximum working life.

Pattern of use is very important here. If your Mac is in a separate workplace, you may only use it from Monday to Friday between 0800 and 1700, say. Although I do go out daily, at least, my desktop system is essentially in use seven days a week between about 0700 or earlier and 2330. Out of the 168 hours that it’s running each week, it could only sensibly be shut down for less than 50 hours, which is under 30%.

Following on from that is the question of how much energy you’d save by shutting your Mac down. This is relatively simple to estimate: in my case I’ve got an iMac Pro and my RAID, which together will use around 100-120 W when idle. So if I shut them down for 50 hours each week, I should save around 5 kW each week, or 250 kW per year, which costs me roughly £45 per year. If my Mac were to be shut down outside ‘office’ hours, say 120 hours each week, instead, then those savings would rise to just over £100 per year.

In the days when Macs had hard disks, that sort of saving was probably less than the cost of replacing disks which failed as a result of accelerated ageing. With hardware savings even tougher to guesstimate now, the answer in terms of cost isn’t as clear.

Finally, you need to know whether your Mac can sleep and wake reliably. This is my third iMac in succession which hasn’t been reliable in this respect, now probably because of the kernel extension for the RAID drive, which tends to be unstable when waking up. When it does sleep, regardless of the Energy Saver pane settings, it also tends to put the drives in the RAID to sleep too, which defeats part of the purpose in not shutting it down.

Some users even question the value of putting a display to sleep: over the working lifetime of a computer like an iMac, never putting the display to sleep is extremely unlikely to cause any persisting faults in its display. Stories about images becoming ‘etched’ into displays go right back to the days of green screen CRT screens, and even then were probably largely urban myths. For the record, though, my never-sleeping iMac Pro does put its display to sleep, if only to stop the cat from sitting up watching its ‘screensaver’ at night.

At the end of the day, it’s a personal decision on the balance of these different factors, and what you feel most comfortable doing.