One cause of computer data loss is the momentary power failure. It’s been said the potential for damage to your computer or the data that’s on it falls into two general categories; damage that has already happened and damage that will eventually happen. If you've ever considered a backup power supply for your computer and not really understood the lingo, or even wondered what size you need, this article may help.

The technical definition of a "UPS" is an Uninterruptible Power Source. These vary both in size (capacity) as well as in quality and features. Most of the cheapies out there serve only as a minimal barebones backup power source while others provide a filtered, pure sine wave output, over voltage & under voltage (brownout) protection, as well as surge protection. As with most any product, features vary and you get what you pay for.

Choosing the right UPS depends on what you expect your standby power system to provide. Most of us would be satisfied with a small unit around 400-VA, just big enough for our PC and monitor, to save us from those annoying momentary flickers or allow us to perform a quick orderly shutdown if the power happens to stay off for more than a few seconds. Other computer “addicts” might want to be able to continue using their computer for as long as possible. The difference is in the VA ratings, how much load you plan to put on it and of course, your budget. A small unit sufficient to carry you through those brief momentary flickers and keep you going for a couple of minutes during a brief outage can actually be found for less than $50. Expect to pay more for higher capacity and longer run times. There are also 3 general types;

the basic UPS with little or no surge protection and no brownout or over voltage protection which simply switches over and picks up the load in the event of a power interruption;
an advanced model which adds over voltage protection and
High-end professional grade systems which provide pure sine wave and constant voltage output (brownout & over voltage protection).

You’re apt to find little price difference between the “a” and “b” variety, so when shopping, always look for one that includes surge protection built-in. The good news is they’re becoming very affordable. Surge protection is expressed in “Joules” and the more, the better. You’ll want at least 800 Joules of surge protection.

The small $50 variety UPS will operate most home computers (CPU and monitor only) for only a couple of minutes. If you plan on including your printer and a small desk lamp, or need a little more time, you'll need something a bit larger than the bare bones model. For the average computer user, a 600 VA (about $100) model will provide around 15 minutes run time (depending on actual load). Just remember, size (capacity) equates to cost. My 1400 VA model cost $400, but it will keep my entire desktop going for over an hour. That includes a small desk lamp, two separate 2 Ghz towers, 19" LCD monitor, flatbed scanner, router, DSL modem, printer, powered speakers and the answering machine. (I really hate having to reprogram my answering machine)

Watts vs. Volt-Amperes (VA), what do the numbers mean? I’m not going to get technical here, buy many years ago everything to do with electrical power was expressed in watts. This made it easy for the layperson to understand because we could all relate to the various wattage appliances and light bulbs in our homes. But then somewhere along the way electrical product manufacturers started playing the numbers game, expressing things in a brand new term, "Volt-Amperes" or just VA. Unfortunately, watts and volt-amperes are not interchangeable terms. You’ll need to know the approximate average conversion factor (1.6). What this means is 100 VA equals approximately 60 watts. That’s not precisely accurate, as there are other things thrown in to complicate the formula, but still using a conversion of 1.6 should get us inside the ball park.

Complicating things, nowhere on the back of your computer or monitor or desk lamp, etc. will you find a VA rating. All of the appliances, computers, etc. in our homes and businesses are still rated in watts.

A good rule of thumb when selecting a UPS is to buy twice the capacity you actually need. This is for two reasons; First of all, having extra capacity ensures that we're always operating down in the comfort zone, well within the sweet spot of the manufacturer’s design curve. Secondly, it gives us that extra margin to allow for plugging in something extra that we overlooked or maybe adding something later. When sizing your UPS requirements, add up all of the wattage ratings of everything you plan to plug in, then double it and multiply that total by 1.6. This will give you the "VA" rating of an appropriately sized UPS for your application with plenty of reserve. If your math works out to be somewhere between two UPS models, opt for the larger of the two, affordability notwithstanding.

Exercise the battery? No, never. The battery experts say not with a UPS. However, purchasing and installing a UPS doesn't necessarily mean that you'll always have reserve power available. Everyone knows that all batteries eventually fail. However, there’s really nothing we can do to extend the life expectancy of our UPS. In my experience I have found that the capacity of my hefty 1400-VA UPS dwindles down gradually over time and loses about 20% of its reserve capacity (run time) per year. In fact, batteries in many UPS systems sometimes fail a lot sooner than expected due to over use. A UPS is not a portable power system like a generator. Rather it’s an “emergency” system designed to be used only in brief, intermittent situations. Certain rechargeable batteries like those used in cellular phones need to be exercised. However, this is not the case with the sealed lead-acid (or gel-cell) batteries used in a UPS. The battery in a conventional UPS is very similar to a car battery, which can easily be damaged by deep-cycling (running it all the way down). We also know that our car battery will eventually one day fail to start our car, regardless of how we baby it. Preventing this inconvenience means periodic replacement of the battery before it fails.

Replacing the battery (or batteries) in a UPS system can sometimes be a daunting task. Some models have an access panel on the bottom making it relatively easy. Alas, I’ve never found one of these easy-access panels on many of the inexpensive models and they’re not that common even on the more expensive models. It’s obvious that battery replacement wasn’t on the top of the manufacturer’s design criteria. In fact, in some UPS units it is clear the manufacturer never intended for the battery to be a “user-serviceable item”. However, with varying amounts of manual dexterity, muscle and some common sense electrical safety precautions, all UPS batteries can usually be replaced for about 1/4 of the replacement cost of a whole new unit, provided you can do it yourself. By the way, finding a replacement battery isn’t always easy. Around Houston, Fry’s Electronics and Altex carry them, so does Interstate Battery. Just avoid surplus/salvage stores

In closing, I don't want to recommend any specific brand UPS in this article. If you’d like to email me I’d be happy to point you toward my personal favorite. Actually they're all pretty good and vary mainly in features (and cost). Also, when shopping for a UPS you may notice that none of the stores selling them carry replacement batteries. That’s no accident. The reason I'm told is that there is very little market for the batteries. They can sometimes be difficult to replace and the task of replacing batteries brings with it some EPA concerns regarding disposal.

Bill Garfield, wdg@hal-pc.org