Off-Line or Standby UPS consist of a basic battery, charger and a switch that senses irregularities in the electric utility. The computer is usually connected directly to the utility that serves as the primary power source, and power protection is available only when line voltage dips to the point of creating an outage. Come Off-Line UPS do include surge suppression circuits, and some possess optional built-in power line conditioners to increase the level of protection they offer.

In the case of power surges, an Off-Line UPS passes the surge voltage to the protected system until it hits a predetermined level, around 115% of the input voltage. At the surge limit value, the unit then goes to battery. With high-voltage spikes and switching transients, they give reasonably good coverage, but not the total isolation needed for complete input protection. For power sags, electrical line noise and brownouts, off-line UPS protect only when the battery is delivering power to the protected system.

A similar limitation exists in the case of frequency variation. An Off-Line UPS protects only if the inverter is operating and on battery. If the input frequency varies outside the device's range, the unit is forced to go to battery to regulate the output to the computer. In very unstable conditions, this may drain the battery making it unavailable during a blackout.

Line interactive UPS are hybrid devices that offer a higher level of performance by adding voltage regulation features to conventional standby designs. They are most effective in protecting against average high-voltage spikes and switching transients.

Typically, Line-Interactive UPS switch to battery to adjust the tap location down at certain intervals to maintain the output voltage as the input voltage changes. When the unit reaches a per-determined point, it eventually goes to battery full-time. More advanced Line-Interactive UPS provide a level of voltage regulation without engaging batteries and they deliver tighter output voltage windows than their predecessors. A unit that is able to deliver consistent, regulated output voltage in a window of -10 percent to +6 percent meets (or exceeds) the operational guidelines recommended by many UPS manufacturers.

Switching to batteries on an as-needed basis means that Line- Interactive UPS are able to operate more efficiently, but it necessitates that batteries be constantly recharged in a timely manner. Recharging happens in one of two ways - by using a standard trickle charge method or a proprietary three-stage charging technique called ABM^TM (Advanced Battery Management). Batteries that are constantly trickle charged tend to reach the end of their useful life more quickly than those using ABM. In addition to nearly doubling battery service life, ABM optimizes recharge time and provides up to 60 days advanced notification of pending battery failure. Line-Interactive UPS are frequently used to protect a variety of networking devices, from hubs and routers to servers.

True On-Line UPS provide the highest level of power protection and are the ideal choices for shielding your organization's most important computing installations. This technology uses the combination of a double-conversion (AC to DC/DC to AC) power circuit and an inverter, which continuously powers the load, to provide both conditioned electrical power and outage protection. Online UPS offer complete protection and isolation from all types of power problems - power surges, high-voltage spikes, switching transients, power sags, and electrical line noise, frequency variation, brownouts and blackouts. In addition, they provide digital-quality power not possible with Off-Line systems. For these reasons, they typically are used for mission-critical applications that demand high productivity and systems availability.

While the purchase price of an True On-Line UPS may be slightly higher than those employing other technologies, it's important to recognize the overall value of comprehensive power protection. True On-Line UPS provide the same benefits as other leading UPS technologies and they offer line conditioning and surge protection, typically at a price lower than the combined cost of a UPS, a line conditioner and a surge protector.