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Standby UPS / Standby Power Supply
The standby UPS is the simplest and least expensive UPS design. In fact, some don't even consider a standby UPS to really be a UPS, calling it instead a standby power supply (SPS). However, many of the most common consumer-grade devices marketed as UPSes, particularly on the lower end of the budget scale, in fact use this general design. They are sometimes also called offline UPSes to distinguish them from online UPSes.
In this type of UPS, the primary power source is line power from the utility, and the secondary power source is the battery. It is called a standby UPS because the battery and inverter are normally not supplying power to the equipment. The battery charger is using line power to charge the battery, and the battery and inverter are waiting "on standby" until they are needed. When the AC power goes out, the transfer switch changes to the secondary power source. When line power is restored, the UPS switches back.
While the least desirable type of UPS, a standby unit is still a UPS and will serve well for most users. After all, if standby UPSes didn't work, they wouldn't sell. For a very critical function, however, such as an important server, they are not generally used. The issue with a standby UPS is that when the line power goes out, the switch to battery power happens very quickly, but not instantly. There is a delay of a fraction of a second while the switch occurs, which is called the switch time or transfer time of the UPS. While rare, it is possible for the UPS to not make the switch fast enough for the PC's power supply to continue operation uninterrupted. Again, in practice this does not normally occur or nobody would bother to buy these units. Still, you should compare the unit's transfer time to the hold (or holdup) time of your power supply unit, which tells you how much time the power supply can handle having its input cut off before being interrupted. If the transfer time is much less than the hold time, the UPS will probably work for you.
Standby UPSes are usually available in a size range of up to about 1000 VA.
Warning: If you use a
standby UPS, make sure it incorporates surge suppression and filtering features for when
the machine is running off standard power, as shown in the block diagram above. Otherwise,
under normal cases (i.e., any time you aren't experiencing a blackout) your system is, in
essence, plugged directly into the wall.