Power interruptions from storms, brownouts, and other glitches lasting only a few seconds or less can wreak havoc on a network. Even the shortest outage can potentially stop important applications and cause an administrator to have to manually reset a router. This is why it's so important to have an uninterruptible power supply (UPS). Unlike a backup generator or other emergency power system, a UPS is designed to kick in immediately, keeping the power supply continuous, by using stored energy, such as from a battery. This can give you time to switch to using a generator in case of a more extended outage.
An online UPS is even better, as this kind of UPS shields equipment from even minor power fluctuations. Since the price for the higher-end UPS models has come down, there's little reason not to use them to protect any important network, especially since they typically have a longer battery life as well.
When buying a UPS, make sure to get the right size for the equipment you're protecting. Vendors have online wizards to help you find select the appropriate UPS, such as at www.apc.com/template/size/apc/cisco/index.cfm. While such sites are designed to sell specific products, you can still use them to get a general guideline for what size UPS you'll need.