There are two basic types of UPS systems: standby power systems (SPSs) and on-line UPS systems. An SPS monitors the power line and switches to battery power as soon as it detects a problem. The switch to battery, however, can require several milliseconds, during which time the computer is not receiving any power. Standby Power Systems are sometimes called Line-interactive UPSes.

An on-line UPS avoids these momentary power lapses by constantly providing power from its own inverter, even when the power line is functioning properly. In general, on-line UPSs are much more expensive than SPSs.

Comparison of power protection methods
Compares the level of protection offered by different types of devices, such as UPS's, line conditioners, and surge suppressors. This page is from "The PC Guide".

Information about batteries used in computer UPS
Offers specifications, resources, FAQs, and cross reference information about UPS and Sealed Lead Acid (SLA) batteries.

PC external power and power supply reference
A reference guide looking at the power systems of the PC from the inside and out. Covers electricity basics, power problems, UPSs, power supply functions, form factors, and sizing and output.

PC Guide's UPS page
Provides an overview of UPS's, including sections on standby UPS's, Online UPS's, and battery service time.

Troubleshooting Power Sources and Power Protection Devices
Lists some typical problems related to the system power, including external power problems, power protection devices (surge suppressors, UPSes, etc.) and the internal power supply. This page is from "The PC Guide."

UPS FAQ
Explains what a UPS is and how it works as well as specific manufacturer information.

Yahoo!'s power supply companies page
Yahoo!'s directory of power supply companies.