"That equipment needs to be connected to emergency power" is a common direction from the client during the design phase of a project. However, it's not always that simple.
While we tend to refer to all of the electrical circuits served from an on-site source as "emergency power," the National Electrical Code (NEC) establishes separate categories of electrical systems intended to supply an alternate source of power on interruption of the normal supply. These are the emergency, legally required standby, and optional standby systems. It is important to know the difference between these systems and to be aware of what equipment may be served from each.