My suggestion for an Uninterruptible Power supply for you computer is not to go out and buy a commercial unit. Why? Because a good one that will support about 300 Watts for more than 10 minutes will cost you around $250 and they are not truly Uninterruptible. In fact they let you computer operate from the power line; then, when the line voltage drops to a preset amount they switch over to their tiny internal battery.

This switch over is not instantaneous but does take some time. So you computer does see a significant glitch in the power. Fortunately, most computer power supplies have enough reserve stored in capacitors to successfully keep running with this glitch. But, what about other devices such as: modems, printers, even monitors, etc.? Will they tolerate such an outage? And even if they do, there remains the question: How long will the internal batteries keep the power up?

The commercial units generally use Gel Cells inside. Gel Cells have the nice feature that they do not spill if tipped upside down, in fact they don't have liquid that can spill. And, they are (unfortunately) available in very small sizes. Both these features make them very attractive for the manufacture of the Uninterruptible power supply, but not for you. Second, because they are very special batteries and are inside it is difficult and expensive for you to replace them. In medical equipment that uses Gel Cells they are typically replaced annually, but it would probably be simpler and cheaper for you to replace the entire power supply. But are you going to do this? Probably not? And even if you did, you would still be using batteries that are very undersized.

Fortunately, in most areas of the United States power failures are rare, but they do occur a few times a year; and with the prospects of "brown outs" at peak power periods, things will get worse. None the less, this makes it so manufactures can sell you something very inadequate; you feel comfortable and probably will never know the difference, until it is too late. That is, until a couple years have passed and you then find out that you have been living with a faults sense of security. Hopefully, you will be able to recover without too much real loss.

I think a much better solution is: To use an ordinary automobile battery, a battery charger, and an inverter; and then run your computer from the AC of the inverter. There is no switch over when the lights go out, and you are immune to AC mains fluctuations such as: complete failures, brown outs, spikes etc. But, unless you are willing to be constantly monitoring the battery status, you will need a charger that is much better than the ordinary ones sold to quickly recharge a dead car battery.

The best battery charger I know of, is a StatPower TrueCharge 20. They are available from West Marine, or other places on the Internet; and will cost you around $275. Yes, it is expensive, but for continuous use it is what you really want. Second, buy a StatPower inverter. (Am I loyal to StatPower? You bet. Do I have an ax to grind? No, they don't know I am writing this; they know that I have sent in the registration card for the items I am recommending. That is all.) The inverters come in a variety of sizes, but for a computer the Port-a-Watt 300 for about $65 should be more than adequate. Third, you will need a battery.

The best batteries are "deep discharge" which will tolerate being discharged and recharged much better than the average automobile battery. But, for this application it would rarely be discharged at all. A good car battery that would fit in your car would be very practical and would eventually serve as a replacement in your car. However, if your battery will only be for this use then look at the deep discharge marine batteries, and of course the bigger the better. (I actually use two 6 Volt golf cart batteries, but then I have a StatPower ProSine 1000 watt inverter so I can run my refrigerator and some lights for 10 to 15 hours if needed. This inverter is great but expect to pay the better part of $800 for it.)

As a side note the computer I am using right now has a Pentium III running at 450 Mhz and an IBM G50 Color monitor, with an EPSON printer and external modem; all on, but the printer is idle. The DC current load (displayed on my inverter) is fluctuating between 11 and 12 Amps. When the display goes into "sleep" mode or when I turned it off and left the computer, modem, and printer on and running DEFRAG (so the hard drive is very active) the current dropped to 6 Amps. This means that with a 100 Amp Hour battery you could run your computer system about 8 hours from the battery.

The advantages of this system are: First your computer is running from the batteries all the time so there is no switch over when the power goes off. Only the charger goes off when the lights go off. Second, you have a much bigger battery which will actually do the job; and you can replace it much cheaper. (Replacement batteries for a lap-top or cell phone cost more than a car battery that has about 100 times the capacity. Furthermore, they are popular and readily available; just try getting a replacement battery for an Uninterruptible power supply.) Third, you can specify and/or change the size inverter or battery. If you want you can take the inverter in you car to run lap tops, recharge cell phones etc. without having to buy cigaret lighter adapters for these devices.

Are there disadvantages? Yes. It will take a little time on your part. The charger wants to be mounted on a wall or bulkhead. I mounted mine on a "Shelf" board that I got in a hardware store and hung it with the charger to the computer table. You will need some heavy, 10-gauge or bigger wire to connect the charger to the batteries. (I bought at Wal*Mart the cheapest ($4.95) jumper cables. That gave me 12Ft. of 10 Gauge double wire and two sets of heavy battery clamps.) Smaller inverters come with a cigarette lighter plug, you can either cut this off and connect it to the battery or you will need to get a heavy duty cigarette lighter socket to plug your inverter into. I got mine at West Marine. Last, you have to find/make a place for the battery and inverter. But, you can be proud of your job when you are done.

If you decide to go with a much bigger inverter you will need some very heavy, like triple zero gauge, cables to be able to supply the current needed for 1000 watt loads. Then you use can bigger things when the lights go out, but this is a whole new discussion.