An uninterruptible power supply (UPS) is a critical component of any computer system in areas where power losses happen frequently or without warning. These battery backup devices ensure that no open files are damaged or lost due to an unexpected power surge, brownout or interruption of service. Backup power units come in many sizes and have varying run times depending on the electrical load generated by the computer(s) that they are connected to. Sizing a UPS for your individual needs is a simple task.
Take a complete inventory of the devices you want to connect to the UPS. Make sure you know what processor you have and what voltage your devices run on. In the United States this will generally be 120 volts. In Europe and elsewhere it is usually 220 volts.
Use an online calculator to determine the best UPS for your needs. This is the easy way to figure out your power requirements. Several UPS vendors offer sizing guides online:
APC: (http://www.apc.com/tools/ups_selector/index.cfm) Tripp-Lite: http://www.tripplite.com/en/products/selectors/ups/index.cfm?gclid=CISHqPec-6QCFUtJ2godHXPtiA Dell: http://www.dellups.com/byDevice_01.asp?ByLoad=Configure+by+Devices
Manually calculate the UPS. Figuring the best UPS for your needs involves simple math. All UPS units have a voltage and amperage rating commonly referred to as "VA." To determine the VA necessary for your own needs, look at the nameplate on each piece of equipment you want to connect to the UPS. Find the voltage and amperage. They will be listed in the format 120V and 3.5A and will vary based on the power draw of the equipment. Multiply volts by amps to get each device's VA and then total the results for all devices.
Add 25 percent to the total VA number. Shop for a UPS that is rated for that VA number or higher.