UPS Sizing made simple:

Basically, the first step in sizing a UPS is understanding what you want to use the UPS to protect. To ensure proper operation of your UPS, you must first size your load. The most common method of sizing a load is the VA (volt amp) measurement.

Figuring your VA Load:

The first step is to identify all the devices that you want to plug into the UPS. One thing to keep in mind is that the UPS must have enough battery protected outlets to accommodate all your devices. Next, add up the wattage of each device. If you are protecting Cisco network equipment, you may refer to the RefurbUPS Cisco UPS sizing page here:

CISCO APC UPS sizing chart

Most devices will have the wattage rating printed on a label on the unit, or on the power supply transformer. The typical desktop computer consumes between 150 and 350 watts, depending on the power of the PC. A typical 19” CRT display uses about 100 watts, and a 19” LCD uses about 40 watts. Below are examples of typical desktop applications:

After identifying the components that you need to protect, add up the VA and Wattage rating for all of them, then Shop for a UPS that is rated to handle that much load. The rule of thumb is to get a unit rated at twice the VA of your basic load, which will extend outage runtime, as well as extend the serviceable life of your batteries.