Uninterruptible Power Supply (UPS) - a low cost, high quality solution by Jerry L.

Loss of power or a brown out to a computer can be disastrous if it happens while writing to a hard disk. It can corrupt the Table of Contents, making the disk unusable without heroic measures. So a computer should have a UPS feeding it to eliminate that danger. Kind of like having house insurance, may never need it but if you do need it, you REALLY need it.

All of the inexpensive UPS units are very sensitive to the quality of the power feeding it. They deliver the incoming line current until that is interrupted then it switches to the battery and inverter which converts battery's 12 volts DC to 110v 60 cycle AC. That isn't a problem unless you have a back-up generator. There is about a 30 second delay from the loss of power and having the generator kicking in. So an automatic backup generator still needs a UPS. The generator's frequency normally isn't right on and the wave form is noisy. The USB doesn't recognize that the generator is running so it continues to run off of it's internal battery, beeping all the time.

The best USB units always run off the battery and are very tolerant of the frequency and wave form of the power source. They are basically just a battery charger, a lead-acid 12 volt battery, and a good quality pure sine wave inverter. For many hundreds of dollars.

When the battery in my cheap UPS bit the dust, I couldn't see buying a new battery for a unit that really doesn't do what I want. I happened to have a battery charger and a spare battery, the type used in a garden tractor. So all I needed was the inverter. I want to power only the computer, any external disk drives that are normally connected, and the wireless router. Those measured about 60 watts total. So I got a commercial grade150 watt (200 watts peak) inverter for about $150 (Samlexpower SA-150 Pure Sine Wave Inverter), wired it up with the battery charger and battery and it works great. The battery charger puts out about 5-6 amps of 12 volt DC, capable of 15 amps so it's not taxed.

What you don't get with this scheme is the ability to automatically shut the system down if the battery runs low. But since, in my case, the generator will be taking over after a few seconds, that isn't a problem. I haven't calculated how long the battery will last since that isn't a concern in my situation. It can be anything you want, just have a larger (or smaller) battery.

The laser printer is powered by the generator in the case of a power failure, not the UPS, because of the amount of power it takes, but that 30 seconds without power can't cause any disaster other than having to reprint something that was interrupted.