Mission critical facilities are broadly defined as containing any operation that, if interrupted, will cause a negative impact on business activities, ranging from losing revenue to jeopardizing legal conformity to (in extreme cases) loss of life. Data centers, call centers, hospitals, manufacturing processes and military installations are the more common types of buildings that could be considered mission critical.

The role of an uninterruptible power supply (UPS) in these applications is clear; there must be a temporary power bridge to support the load between the loss of utility and the transfer to diesel generator.  The computerized equipment such as server, storage and network devices that run these processes are sensitive to fluctuations in power quality. As a solution, these fluctuations are mitigated with the use of a UPS system.  These UPS systems can be divided into two broad categories based on their method of energy storage: