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You can't specialize in everything, so don't feel ashamed or embarrassed that you don't understand the differences among UPSs and how they "behave" in protecting (or not protecting) your equipment.  If you're an engineer, you might wanna skip this page because it's written so that an untrained IT person can understand it, and help him or her make a more informed decision in UPS purchasing. There are basically 3 types of UPSs: Stand By, Line Interactive and Online. Different manufacturers designate them differently in their product lines.  For example, Powerware calls them Level 3, Level 5 or Level 9 protection. Let's look at each one. STAND BY UPS (also called Off Line UPS) This guy does just what its category name implies.  It just sits there ignoring everything that's going on with your power, except a complete power outage.  They typically have a stepped (fake) sine wave output.  The cheapest- of-the-cheap stand by units will do only that, turn on when the power goes out. More expensive ones may also offer some surge protection, possibly even some automatic voltage regulation (AVR).  A stand by UPS can take as long as 35 to 45 milliseconds to switch over to battery power.  They are the least protective of attached equipment and should only be used with equipment that is not all that important, and that may also be attached to a surge protector. Examples of Stand By (or Off Line) UPSs:      APC BackUPS      Powerware units beginning with the number 3      Liebert PowerSure PST and PSP lines LINE INTERACTIVE UPS The next higher level of UPS protection is line interactive.  These units (with the possible exception of "off brands") will offer a quicker response time (2 to 10 milliseconds to go to battery) and have some measure of automatic voltage regulation (AVR) and surge suppression.  Lower-priced units will put out a stepped, or modified, sine wave, but higher-priced units will produce a pure sine wave.  Also, higher end units will go to battery in a brown-out (to boost the voltage) and in an over-voltage situation (to lower or "buck" the voltage).  [So, if you hear the term "buck and boost" you'll know what's being talked about!]  The best line interactive units will perform buck and boost without having to go to battery and thus extend your UPS's battery life.  Some line interactive units also offer a settable range of voltages they'll tolerate before going to battery. Examples of Line Interactive UPSs:      APC SmartUPS      Powerware units beginning with the number 5      Liebert PowerSure PSA and PSI lines ONLINE UPS The highest level of UPS protection is online.  Online units take the incoming voltage, rectify it and downconvert it to the battery voltage of the UPS, filter it, reinvert it and upconvert it to the appropriate output voltage for the unit. The output voltage is always rock-steady and the sine wave is always pure.  Nothing bad (power-wise) gets through an online UPS.  In addition, there is ZERO transfer time to go from failed line voltage to battery power.  If power fails, you're just "there"!  Online units also tend to take better care of their batteries, so you'll typically get a year or two more lifespan from them over the other types of UPSs. Examples of Online UPSs:      APC SmartUPS RT Series, Symmetra      Powerware units beginning with the number 9      Liebert GXT and GXT2 lines, Nfinity      Most 3 phase UPSs are online units, regardless of size or manufacturer   As with most everything else in life, so with UPSs, you get the level of protection that you pay for. For absolute, non-fail, ultimate protection of your equipment and data, spend a little more and get an online UPS. Because of its non-varying output, the electronics you attach to it will last longer, whatever they are. Oh, and one other piece of wisdom:  Before you become enamored with the "$30,000 equipment protection guarantee" (or whatever the amount is) read the fine print.  You'll find it's loaded with "weasel words" that let the manufacturer out of it.  For example, a UPS is designed to fail in taking a lightning hit to protect your equipment.  A lightning strike is not covered.  Your UPS just did its job.... I hope this page has helped enlighten you to the goings-on in the UPS world.  If you have any questions or comments (or corrections!), please write, IM, live chat, or call me.  I look forward to hearing from you! Bruce or IM me at AOL Instant Messenger (AIM):  Bruce 3PG   Unless otherwise noted, the units and prices listed in this website are for reconditioned or open box units. "It's better to be a year too early than one minute too late." Order from: Bruce Hopkins B P S F 1715 CR 2540 Quinlan, Texas 75474 (903) 751-0921 (903) 356-6233 - fax http://www.secure-power.com . . . Rev. January 31, 2008 § This life courtesy of the grace and mercy of our Lord and my Savior, Jesus Christ.