Powering your Mac: power filters and UPS

When you next shut your Mac down, time how long it takes between choosing the command and its screen going black. Although not as long as starting up, there’s a lot more to shutting down than even meets the eye. Now look at where your Mac gets its mains power from: if it’s plugged straight into a mains power socket, ask how long loss of mains power gives your Mac to shut down. It doesn’t, unless your Mac is powered through an Uninterruptible Power Supply (UPS). This article is about why you need a UPS, and why you don’t need to spend a fortune when buying one.

Like all good electronic equipment, Macs have a power supply which converts high voltage AC mains to the lower voltage DC supplies required by chips and all the rest of its hardware. Although Mac power supplies are among the best in the industry, they’re still susceptible to surges and drop-outs in the mains supply, and they can’t provide power when the mains is lost altogether, of course. Providing your Mac with a reliable filtered AC supply is very important to protect it from sustaining damage during electrical storms, when there’s utility maintenance underway, or some idiot trips a circuit-breaker or pulls the mains plug.

The major exceptions to this are laptop models – MacBook Pro, MacBook Air, and MacBook – as their internal batteries provide their own backup power. As a result, if you do connect one of those to a UPS, it won’t be compatible with the USB connection which would otherwise be used to trigger shutdown when the mains supply is lost. Laptops still benefit from the filtered mains supply from a UPS, though.

Good UPSes are more expensive than mains power filters, but filters don’t provide any backup power. When the mains power is lost, even for a few seconds, all they do (when they work properly) is stop your Mac from being exposed to voltage spikes which can be most damaging. You get what you pay for, I’m afraid.

The commonest error in deciding whether to use a UPS is the argument that, because your Mac isn’t left on 24/7, it’s always ‘attended’, so should anything go wrong with the power, you’ll be able to deal with it. Even if you’re sat at your Mac, with instant reactions, there’s no way that it can shut down in time to protect it. Whether you use your Mac for half an hour a day or only power it off once a year for cleaning, it needs a UPS.

Next in the reasons we persuade ourselves to believe is that UPSes are expensive. Yes, many are, but the more expensive ones are designed to keep things like power-hungry servers running for an hour or more. Most Macs are well-protected if the UPS keeps them going long enough to allow an orderly shutdown – a minute or two at most. It’s far better for a Mac to be given that chance than to have no UPS at all.

So what do you need in a UPS?

First and foremost, it needs a USB interface which you can connect to your Mac. There are two ways that a UPS can tell your Mac that it has lost mains power (or there’s another reason for imminent loss of power, such as failure of the UPS itself): via USB or over a network using SNMP. As Macs don’t come standard with the latter, which is primarily intended for servers, the USB interface is your only real choice. When that’s connected to your Mac, the Energy Saver pane should recognise it and offer both display options and Shutdown Options, which determine how quickly your Mac shuts down when mains power is lost and it’s running off the battery in the UPS.



There’s an important fact which can sometimes be forgotten: the USB interface on a UPS can only be connected to one Mac. If you’ve got two Macs to protect, they each need their own UPS, as one UPS can’t tell two Macs to shut down, except by SNMP, which is considerably more complex to use.

The other critical feature of your UPS is that it’s capable of delivering sufficient power to run your Mac, connected peripherals such as external disks, and the displays normally attached to it, for long enough to allow their orderly shutdown.

UPS manufacturers like APC (whose UPSes I invariably use) provide online calculators to work out which models are most suitable. Here are a couple of worked examples.

My iMac Pro is an old Intel design which is demanding of power, with a maximum of 370 W. To that I add a couple of external SSD enclosures which might draw another 30 W at the very most, bringing the total to 400 W. To keep that going for 5 minutes, a 900 W (1600 VA) UPS should be sufficient, and capable of exceeding 9 minutes. I could go as low as 500 W (750 VA), but then I’d really want the Mac shut down in 2-3 minutes to be safe.

My M1 Mac mini is a new Apple Silicon design which draws a maximum of 39 W, little more than 10% of the iMac Pro. Coupled with an LG 4K 22-inch display drawing a maximum of 120 W, total power is probably no more than 150 W. Even a 200 W (350 VA) UPS would run that happily for more than 10 minutes.

Translated into cost, a decent UPS with USB interface for my iMac Pro is likely to cost me a little more than £/$/€ 200, while that for my M1 Mac mini need be no more than £/$/€ 120.

The APC UPS selector tool (UK localised) is here.

There are two additional needs when you’ve got a UPS. The first is to check that it all works every few months. I was nearly caught out once when there was a real mains power outage and macOS didn’t shut my Mac down in the time that I expected. Fortunately, I was present and shut it down myself. Had that occurred in the middle of the night, I would have come down in the morning to a UPS with a flat battery, and a Mac which had suddenly lost all power.

The other is to keep an eye on the battery in the UPS. Like all batteries, they eventually wear out. While manufacturers and sysadmins may recommend routine replacement every three years or so, they should be fine for quite a lot longer. Most good UPSes inform you of their battery condition. When it needs to be replaced, you can then go through the agony of deciding whether to pay a high price for a manufacturer’s replacement, or buy a new UPS. But that won’t be for a long time yet. As I recall, my last UPS survived two or three different iMacs in succession before its battery gave up the ghost.