Can you give me some more information on this? E-mail
The UPS industry is made up of many manufacturers, and there is a lack of standard terms within the industry. I think this sometimes borders on deliberate misdirection. (It's a jungle out there!) [ Note, in recent years the whole industry seems to have gotten better, at least mostly agreeing on what the terms listed here mean. This is not true everywhere, but things are getting better. ] 

There are basically three different types of devices, all of which are occasionally passed off as UPSs. 

Standby power supply (SPS). In this type of supply, power is usually derived directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary, when line power is available. This type of supply is sometimes called an "offline" UPS. 
The quality and effectiveness of this class of devices varies considerably; however, they are generally quite a bit cheaper than "true" UPSs. The time required for the inverter to come on line, typically called the switchover time, varies by unit. While some computers may be able to tolerate long switchover times, your mileage may vary. [ Some articles in the trade press have claimed that their testing shows that modern PCs can withstand transfer times of 100ms or more. Most UPS units claim a transfer time to battery of about 4ms. Note that even if a computer can stay up for 100ms, it doesn't mean that 100ms switchover is okay. Damage can still be done to a computer or data on it even if it stays up. ]

Other features to look for in this class of supplies is line filtering and/or other line conditioners. Since appliances connected to the supply are basically connected directly from the power line, SPSs provide relatively poor protection from line noise, frequency variations, line spikes, and brownouts. 

[Most SPS's claim to have surge/spike suppression circuitry as well as transformers to "boost" voltage without switching to the battery if a modest voltage drop occurs. This, it is claimed, allows operation of the equipment indefinitely under brownout conditions as long as voltage does not drop below the lower cutoff voltage.] 

Hybrid [ or Ferro resonant] UPS systems. 
The theory behind these devices is fairly simple. When normal operating line power is present, the supply conditions power using a Ferro resonant transformer. This transformer maintains a constant output voltage even with a varying input voltage and provides good protection against line noise. The transformer also maintains output on its secondary briefly when a total outage occurs. Manufacturers claim that their inverter then goes on line so quickly that it is operating without any interruption in power. Other UPS vendors maintain that the transition is less than seamless, but then again it's not in their best interest to promote CVT based UPS products. CVT based UPS has a sizable part of the UPS market. 

[ Note: According to some sources, Ferro resonant transformers in an UPS system can interact with Ferro resonant transformers in your equipment and produce unexpected results. On the other hand, Ferro resonant UPS systems don't kick off a lot of heat, which is important in some environments. The Moral: Again, test before you buy. ] 

What I call "true" UPS systems, those supplies that continuously operate from an inverter. Obviously, there is no switchover time, and these supplies generally provide the best isolation from power line problems. The disadvantages to these devices are increased cost, increased power consumption, and increased heat generation. Despite the fact that the inverter in a "true" UPS is always on, the reliability of such units does not seem to be affected. In fact, we have seen more failures in cheaper SPS units. [ Note, though, that given the same quality inverter, you'd expect the one that runs least to last longest. ]
 
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