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An uninterruptable power supply (UPS) is essentially a power inverter (like you would use to convert the 12 volts from a car battery to 115 or 220 volts AC for home appliances). The UPS also contains a battery and charger to keep the battery charged and ready for a power failure. A UPS is rated for a number of VoltAmps (VA) and Watts. The wattage is the maximum true power demanded from the power grid. To find the maximum current the UPS will demand, divide the power rating by the power grid/mains voltage. The VA rating is the maximum apparent power (AC volts multiplied by amperes) that can be delivered to equipment.
Regardless of the VA and power rating, you are most interested in how long the UPS can deliver power to your equipment during a power failure. This can usually be found in the advertising literature. For example, a particular UPS may be advertised to provide 15 minutes of power at half-load and 5 minutes at full-load.
A UPS usually has some means to communicate with a computer to notify when a power outage occurs (via USB, serial port, etc.) with software to pop up a warning and shut down the computer before the UPS shuts down. Such software may be able to broadcast a message over the network that the server is about to shut down.
A battery backup system is like a UPS but passes the power grid/mains power directly to its outlets. It quickly switches to battery mode upon power failure. This can cause a glitch in the power that can cause unpredictable behavior of computers, so an actual UPS is usually preferred over a battery backup system. Be sure to research any UPS you are considering to be sure it is an actual UPS and not a battery backup system if you require an actual UPS.
A power conditioner compensates for voltage sags power from the power grid that is not a clean sine wave. An actual UPS acts as a power conditioner since it has a power inverter that isolates the equipment from the power grid,
A surge suppressor absorbs voltage spikes. Voltage spikes can occur on the power grid from large motors turning on or off, power failures in adjacent areas, large birds or aircraft striking power lines, or lightning strikes.
Cheap surge suppressors use metal oxide varistors (MOVs) and are unreliable. Once a MOV absorbs a power surge, it is destroyed and cannot absorb any more surges (power companies have large MOVs in critical areas and must check and replace them regularly). Good surge suppressors use inductors and regulator circuits, costing around $50 or more for a low-power unit.
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