When a power failure or brownout is detected, the inverter draws the power it needs from the UPS's internal battery. On-line UPS systems are the most expensive option we've discussed, but they are ideal for sites with chronic power fluctuation problems. What you should use depends on what equipment you have, its function, and your environment. It is our considered opinion that every piece of computer equipment should have quality surge protection and noise filtration. Ideally, the phone line to your modem and any network cabling should also pass through a surge protector.
Some models offer plugs for phone lines and network cables.
You should think of this purchase as a kind of insurance policy on your computer and the integrity of its data. Surge protection and noise filtration is all most people will need. However, if your computer is on the same circuit as heavy electrical equipment such as a refrigerator, you may wish to consider a voltage regulator. If your lights dim fairly often, you should definitely consider one. If you also need protection from power outages, you might consider an on-line UPS instead.