Main Interruptible Power Supply Technologies

Each of the various UPS types deal with multiple power problems. Here's a short summary of the design features and the power problems they handle. In the table, the last three columns indicate which UPS types can have an effect on the particular issue.

Stand-by or Off-line UPS technology (S):

Stand-by or off-line designs are intended to provide a low-cost solution for power failures, power sags and power surges. Utility power is provided during normal operation. Small voltage and frequency changes are not regulated by these units and are passed through to the equipment. When voltage or frequency changes become too severe, the inverter in the UPS converts DC battery power to AC power to run the equipment. Most of these units provide modified square-wave power.
Line-interactive UPS technology (L):
Line-interactive technology provides basic power protection at mid-range prices. In addition to protection against power failures, power sags and power surges, these units protect against under-voltage and over-voltage. This design offers low-grade voltage regulation by "bumping" the utility voltage up or down before passing it through to your equipment. During the UPS voltage changes, the battery-powered inverter may be used for regulation.
Although these units provide more voltage regulation than stand-by or off-line units, battery life is often sacrificed. The number of transfers to and from the battery by the line-interactive design exceeds that of the on-line units by as much as 10 to 1 during brownouts and power surges. Some of these units provide true sine-wave power.
On-line UPS technology (O):
These units are designed to provide complete power protection. They protect against all types of power problems by continuously using the battery and inverter to create 100% new, clean, regulated sine-wave AC power for your systems. Equipment is isolated from all types of power problems when supplied by an on-line, double conversion UPS.
On-line units isolate equipment from harmonic distortion and are strongly recommended for critical applications that must meet the 5% maximum harmonic requirement stated in many equipment manufacturer's specifications and installation guides.
While the inverter is in use all the time, the battery is kept fully charged whenever there is utility power present. These units use battery power less than any other UPS technology.