I basically recommend that anyone who owns a comuter should get a UPS. No exceptions. Given the potential anywhere for weather or an even minor glitch with your local power provider to cause havoc on your AC line, it really makes sense to protect your investment in your computer. Even fluctuations in power that are too quick for you to even notice (i.e. the lights flicker briefly) can potentially cause directory problems, data corruption or even hardware damage.
[robg adds: The following isn't a traditional OS X hint in many ways, but with the coming of winter and its storms, I felt it was worth a bit of space here to provide some basic UPS info. If you have no interest in such topics, don't bother reading the rest of this hint :). This was also a good reminder for me that it's time to replace my aging UPS; it's well past its useful life...]
Here are a few things to consider:
- More load capacity is better. Without actually going into calculating wattage into load, here's a pretty easy guide. If all you have is a slot-loading iMac and an inkjet printer, you can probably get away with a 350VA model. Most G4 desktops will need 400-700VA, depending on the model and configuration. If you have a PowerMac Dual G5 2.7GHz with a large display and a couple of external hard drives, you'll need at least a 1KVA UPS -- bigger if you need more than 10-15 minutes of on-battery runtime. You might also consider getting an extra UPS for your other less-critical peripherals like scanners, inkjet printers, network hardware, etc. If your configuration is in-between, I'd still recommend as high a capacity as you can afford -- just for the extra runtime and load capacity for future growth.
- Look for the acronym AVR (Automatic Voltage Regulation) on the box when UPS shopping. There's a substantial difference between these and regular UPSes. A non-AVR UPS is basically just a battery backup with a good surge-protector built in. This protects against gross spikes in the power and the power going away completely for a predetermined amount of time. But when the power does go down, there's still a sizable gap (in computer time) between when the power goes out and when the UPS decides that it's time to switch over to the battery completely. What AVR does is constantly monitors the line voltage, and it not only suppresses spikes, but supplements even millisecond dips in the voltage (which can be more harmful than the spikes) with battery power to give you a nice smooth sine wave of AC power at the correct level. The upshot is not only does your computer get cleaner power, but if the power goes out the switchover to battery is much faster and smoother.
- Most consumer UPSes have two sets of outlets on them. One set is wired for battery backup and surge protection and the other set provides surge protection only. Make sure you're plugging in the important stuff -- computer, monitor, external hard drives -- into the correct outlets.
- Do not plug a laser printer into a UPS unless you've bought at least a 2500VA UPS that is dedicated to that printer. Laser printers draw a huge amount of current when they kick in and start printing. If possible, you don't even want to plug a laser printer into the same circuit as your UPS or it might complain about low line voltage every time you print.
- Some places in a city's power grid may have chronically low AC voltage -- this usually depends on neighbors or local industry. The acceptable range for AC voltage is generally 104V to 120V. If you plug in a UPS where the voltage frequently dips below 104V, it may beep often and be unable to keep its battery charged up. Likewise in a household, if the UPS is used on a kitchen or workshop circuit where a number of other high-draw devices are used, you'll get the same results. If you know you have constant brownouts, or you buy a UPS and it screams a lot, call an electrician. Keep in mind, though, that there may be situations on a power grid where nothing can be done until the power company upgrades your area. If the power quality is bad enough, a UPS won't do you any good. Most UPSes will also detect wiring faults -- swapped hot-cold, no ground, etc. -- but will usually just tell you that there's a fault, not which type it is. If you want to check this ahead of time, you can buy a little AC wiring checker for a couple bucks at your local hardware store. Pretty much all UPS manufacturers will not honor a UPS's warranty if it's used where there's a wiring fault.
- Brand: Here are the three manufacturers that I'm most familiar with...
- APC is, of course, the market leader. Pros: They make a wide variety of models, all are excellent quality, and they put a good effort into their monitoring software for the Mac. Cons: They tend to be pricier than most, and in my experience, their batteries don't generally last more than a couple years.
- Belkin also offers a good selection, good software for the Mac, and their batteries generally have a good life -- at least three years or more. They can be pricey, but you can also find some good deals occasionally. Bad news is that I've seen quite a few of them (one of mine included) simply die -- not due to dead batteries, but simply because their little brains just gave up the ghost.
- The only bad thing I can say about Tripp-Lite (my old one has been a rock and it's at least four years old) is that they don't make any software for the Mac. However, PowerAlert is Java-based, and they do have a Linux version of the latest release, as well as the Linux source of an older version. One of these may be workable, although I didn't have any initial luck with the Java version -- so for any advanced monitoring or logging, you might have better luck with the older source. Good news, though, is that even their latest model is compatible enough with Tiger that you can just plug it in and see the battery capacity in the menu bar. And through the Energy Saver pref pane, you can set a threshold for shutting your machine down when the battery gets to a certain level, or for when it has been on battery for a certain amount of time. If you're not running Tiger, there are some free and shareware offerings on versiontracker.com which should do the same thing (robg adds: PowerGuardian is one that I found, though I haven't tried it.)
I'm sure many others will have their own input on the subject, so let the replies begin...

