Power Protection

How Much Power Do You Need?

When you go shopping for an uninterruptible power supply, you'll quickly notice that all the vendors list two ratings: volt-amp and wattage. Watts represent the actual power your provider is delivering. VA is typically called apparent power and is derived from the voltage applied multiplied by the current drawn. The main thing to remember is not to hook up more equipment to the UPS than the device can support; your load can't demand more watts than your UPS can deliver.

So how many watts do you need? You can work the answer out by multiplying the amperage rating of every component you want to power (usually listed on the component itself or on the AC adapter) by the voltage (usually 120 volts), and totaling that for all the components. The UPS you choose should have a higher VA rating than your calculated figure.

An easier way to get an idea of probable run time is to use the online UPS selector offered by vendors such as APC, PowerWare, and Tripp Lite. These tools provide a simple way to identify UPS products that might be suitable: You enter the components that you want to protect and how long you want them to keep running, and the selector comes back with several suggested products. Also consider which devices you plug into your UPS's battery-backed sockets. To extend run time, plug in only the core components that will allow you to save your data and shut down the PC, such as the computer itself and the monitor, but not the speakers and printers.

Note that laser printers should never be powered through a UPS--they draw a lot of power when printing and can overload it.