Voltage Stabilizers and UPS units are different devices, and then there are SOME that can do both jobs.
A Voltage Stabilizer can reduce substantially any sudden changes in voltage from the wall mains, whether too high or too low, but can not deal with a change that lasts several seconds or longer. So these devices can protect your system from damage due to high voltage spikes, or from shutting down immediately from a low voltage event. But they can NOT prevent a shut-down from a power failure over more than a couple seconds.
The potential damage from LOW voltage that lasts over a few seconds is that it forces your system to shut down right away. SOMETIMES this just happens to occur at the time that your system is trying to write information to a HDD, and that new info gets corrupted by this interruption. Then it is possible that the corrupted data on your HDD will make it un-readable and that can be a big problem. So, IF that sort of extended voltage sag problem is common in your area, you need a different device to deal with that.
An Uninterruptible Power Supply (UPS) is that device, but there are several types. Basically all UPS's are battery-powered replacement power sources that can provide power to your system for a limited time with almost no interruption when the mains power fails. The time it can maintain power to your system depends on the size of the unit and its battery, and on the load (power consumption) of your computer system. Most UPS's will specify the maximum power they can put out and the length of time it can continue to do that at full load. Many also will tell you other ratings of time to keep running at specified loads less than maximum. So you might see specs of 300 W max power maintained for 2 minutes, but up to 5 minutes of the load is only 100W. The usual way to use these devices is that, when the wall power supply fails and the UPS takes over, YOU are responsible for stopping all current operations, ensuring everything necessary is stored on the HDD or wherever, and then shutting down in a normal orderly shut-down process while full power is still available to your machine. For systems that simply cannot be allowed to shut down and must keep on running for a long time during a power failure, the UPS system is MUCH larger, more complex, and much more expensive, so most home users will never go to this level.
SOME UPS units ALSO include voltage stabilization in their features. I just bought one of those for a small load. The maker also had cheaper units that did NOT have voltage stabilization included, so do not assume that it is there.
There are two other characteristics of UPS units that can be important. One is waveform. The power from your wall outlet comes in a pure sine wave form. Generating power just like that from a battery at high power levels (several hundred watts or more) is more difficult and expensive, so many UPS units approximate that by putting out power that is a series of steps up and down somewhat similar to a sine wave. For may devices using this power (e.g., the PSU in your computer), that is still a completely acceptable power source because they sort of filter and smooth incoming power. But for SOME small devices with simple PSU components, and for some cheap computer PSU units, that kind of power supply is too much trouble. So you have to decide whether your equipment can tolerate that stepped synthetic sine wave, or you have to buy the more expensive designs that put out a "true sine wave".
The last item is called switch-over time. The simpler systems supply power to the output sockets for your computer directly from the input from the wall power mains. They also use that input power to keep their battery fully charged, AND they monitor the input power for failure. When that input does fail, they switch their operation and turn on the output power generation circuits that draw from the battery, disconnecting their outputs from the mains that have failed. This change takes a small amount of time - usually less than a second. For many systems that is entirely OK - the PSU in the computer already was designed to tolerate very short voltage sags with no trouble. But there are some devices that cannot tolerate that, so the short voltage sag may cause it to reset or shut down. If you have equipment that cannot handle such short-term sags, then you need a different UPS design. The more expensive units have NO switch-over time. They ALWAYS supply all the power to their outputs through their battery-powered sine wave generation systems, and they always charge the batteries from the mains supply at the same time. So when the mains supply fails there is not interruption of output at all. What does happen, though, is that now the "clock is ticking" on the limit to how long the batteries can maintain power to the load with no mains input.
You also need to make some important decisions about exactly what devices you need to maintain power on. Many people would use a USP to maintain power to the computer, monitor, and MAYBE a router, an external drive unit and network storage unit. They would NOT attach the printers, modems, FAX machines, telephones, speakers, etc. to the UPS. Those devices CAN be allowed to fail suddenly with no danger to the computer or to the data stored there, and none of them is critical to continued operation of the office in most cases. So you would arrange for TWO sets of power connections - one group from the UPS for the critical components and thus limiting the load on the UPS, and the other group for devices that can be allowed to fail.
I just bought a UPS for a small load unit in your network system - not for the main computers etc. Because the load was small the size / cost of the unit was modest. I chose a unit with PCM synthetic sine wave output, with modest switch-over time because the load could tolerate a short sag, and with voltage stabilization features just for caution since adding that feature did not cost much more. I expect the unit will be able to maintain power to the small load for an hour (maybe) but we have not tested that yet.
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