Several conversations I’ve had with a few people lately have led me to believe that some folks are under a big misconception about what an uninterruptible power supply (UPS) is for.
For example, I had one person claiming they needed a new UPS for their office server. The server itself is an older monster that sucks a lot of power. Now, I had looked over the server a few days before and the battery indicator on the UPS was showing full, and the load less than half (in spite of being a beefy server – the person who sized the UPS did a pretty good job mating it with the server). So I was surprised to hear there was a problem.
“Why do you think the UPS needs replacing?” I asked. “Because we had a power outage, and the UPS didn’t keep the server up until the power came on.” I asked how long the server stayed up. “A couple of hours.” Hours? Sounds like the UPS is working fine. How long was the outage? “3 days.”
A battery-only UPS system is not designed to take the place of your power company! Battery backups provide two main services. First, they bridge the gap when you have small power glitches, either spikes, brownouts, or outright interruptions. A decent UPS will clean up the power and make sure you have a steady, consistent source of energy for your electronics. Second, they give you a chance to shut your equipment down gracefully (or have it do it automatically) if the outage is for more than a few minutes.
Graceful shutdown is very important. Just as important as protecting the electronics physically from being blown out, you want to protect the logical portion of your system. While modern file systems are pretty resilient, there are always “bad times” when a loss of power means data corruption. You could ruin a file or possibly a whole file system on disk if the power is cut at just the right time (and of course, you know that’s when any problem will strike!).
So it’s important that you get some sort of battery backup system for your computer. Most of them these days will have a USB cable that you can connect to the UPS and your system. OS X has built-in software to communicate with the UPS. The UPS can let the system know when there’s been a power interruption, and how long the UPS can supply power. You can then set your system to shut itself down gracefully if, for example, the UPS reports only 5 minutes of power left.
(Update- thanks to James Cutler for the screen shots – my computer for some reason wouldn’t acknowledge my UPS.)
You should buy a unit that can run your system for at least 5-10 minutes. Look the power your system draws (Apple supplies that info under “Tech Specs” at their support site). You’ll need to multiply the volts times the amps listed to get the “VA” (volt-amps) rating used by many UPS units. However, note that Apple lists the maximum amperage – chances are you will not be running at maximum. So take 2/3 or 3/4 of the result to avoid over-buying. Don’t forget to factor in anything else you have plugged in, like external disks. In most cases, they won’t amount to much, but if you have a MacPro with 8 external drives, it adds up.
Sometimes Apple supplies the wattage instead of the amps. Watts are basically volts times amps as well (and before the engineers come down on me, I know that’s not strictly correct except for DC, but it’s close enough for these calculations). Again, these are maximums, so you can go a bit lower and play it safe.
The UPS manufacturer should list, either on the box or on their website, the approximate run time based on the VA or wattage number, for their particular models.
If you expect to be down on a regular basis for more than 5 or 10 minutes, you’re either going to need a big UPS (read: large and expensive) or a generator. I have a whole-house generator that runs most of my house (other than the air conditioner compressor) in the event the power goes out for more than 30 seconds. So the UPS units on my computers just need to bridge that gap.
You shouldn’t expect more than a few minutes out of your battery backup. But you can maximize those minutes by putting as little as possible on the UPS. For example, if you have a printer – never plug that into a battery backup! Some backup units have a section of outlets labeled “surge protection only” – that’s where you can put a printer, or else get a good surge protector. But most printers will suck a battery dry quickly if you attempt to print something. Really, the only things you should have on your battery backup are your computer and any external disks. If you have a separate monitor (as opposed to the built-in one on the iMac) you should leave that on the surge-only side too. Monitors suck down a lot of juice. If you have your UPS configured to automatically shut down the system when the battery gets low, then it won’t matter that you can’t see to close it down. Of course, you still might lose a document you have open (if the OS forces the program to quit while shutting down and you can’t see the screen to do it), so it’s always a good idea to do frequent saves.
A UPS battery backup unit is a very important addition to your computer setup – and it should not be an optional part.
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