A brownout is a ​momentary decrease in voltage; also known as a sag. An overtaxed electrical system can cause brownouts, which you might recognize in your home as a dimming of the lights. Such voltage decreases can cause computers or applications to fail and potentially corrupt data. A blackout is a complete power loss. A blackout could cause significant damage to your network. For example, if a server loses power while files are open and processes are running, its NOS might be damaged so extensively that the server cannot restart and the NOS must be reinstalled from scratch. A backup power source, however, can provide power long enough for the server to shut down properly and avoid harm.